Sedimentology (1 992) 39,567-579

Sediment dynamics and hydrodynamics in the lower course of a river highly regulated by dams: the Ebro River

J O R G E G U I L L E N and A L B E R T P A L A N Q U E S

Instituto de Ciencias del Mar. (C.S.I.C.) Paseo Nacional sln, 08039 Barcelona, Spain

ABSTRACT

The lowest part of the Ebro River is a microtidal salt-wedge estuary. Penetration of the salt-wedge is largely controlled by the fluvial discharge and the morphology of the river bed, although sea level variations caused by tides and atmospheric conditions can also play significant roles. The concentration and distribution of suspended particulate matter in this part of the river and the fluvial sediment discharge are strongly influenced by the dynamics of the salt-wedge. Damming of the river has caused sediment to be trapped in reservoirs and has regulated the fluvial discharge. Intrusion of the salt wedge has thus also been regulated. At present, sediment discharge is between 1 and 1.5 x lo5 tons per year, which is less than 1% of the sediment that the Ebro River discharged into the sea before construction of the dams. This extreme reduction in sediment supply has allowed marine erosional processes to dominate in the delta.

INTRODUCTION

The Ebro River is one of the most important fluvial systems discharging into the western Mediterranean. The river is 928 km long with a drainage basin 85 835 km2 in area, and it has developed a subaerial delta 325 km2 in area (Fig. I). Sedimentary discharge during the Pliocene and Pleistocene has been esti- mated to oscillate between 6 and 15 x lo6 tons per year (Nelson, 1990). The hydrology and sediment discharge of this fluvial system have been dramatically altered by human activity (Palanques, 1987; Pal- anques et a[., 1990a,b). There was intense deforesta- tion of the drainage basin during the fifteenth century, leading to a rapid progradation of the deltaic system. Construction of irrigation systems became important qt the end of the last century and early part of this century, when two important channels were built In the lower part of the river. Many dams and reservoirs have been built during the twentieth century. The area most affected by these changes is the delta system.

This paper focuses on the hydrodynamics and sediment dynamics in the lower 30 km of the Ebro River, where a salt-wedge estuarine circulation devel- ops during certain times of the year. Special attention

is paid to the effects of dam construction and the salt- wedge dynamics on the sediment transported by the river to the sea. The Ebro River provides a represent- ative example of the present sediment dynamics of Mediterranean rivers affected by salt-wedge intrusion (Pauc, 1987) and damming.

METHODOLOGY

Seven surveys were carried out between May 1988 and February 1990. A systematic study was made of three sections of the river located between the town of Amposta and the river mouth (Fig. 2). Water samples were taken with a pump for every 1 m of the water column at each section. Simultaneously, vertical temperature and salinity profiles were recorded using a CTD Aanderaa 3020A and 2975, respectively. The vertical profile of the river current velocity was recorded using a Valeport Braystoke BFMOOI current meter. Some temporal series of current velocity were

‘recorded with Aanderaa RCM-7 current meters. Bottom sediment was also sampled with a grab. The morphology of the river bed was studied using an Ore

567

568 J . Guillkn and A . Palanques

Fig. 1. Drainage basin of the Ebro River and location of the main dams and physiographical elements. M: Mora Depression, P: Priorat Massif.

Side Scan Sonar model 159 instrument and a Simrad EA 300P echo sounder.

The water samples (133) were vacuum-filtered into pre-weighed Nucleopore filters (0-4 pm) and the concentration of the suspended sediment was esti- mated gravimetrically. The size of the suspended particles was analysed using a Coulter Multisizer. The grain size of the bottom sediment was analysed using a Sedigraph 5000-D for the < 50 pm fraction and a settling tube for the > 50 Frn fraction, following the methods described in Giro & Maldonado (1985) and Diaz er al. ( 1990).

GEOLOGICAL S E T T I N G

The Ebro River basin covers several geological terrains : the sedimentary and metamorphic Palaeo- zoic terrains of the Iberian and Pyrenees mountains, the igneous terrains of these two areas and of the

Priorat Massif, the sedimentary Mesozoic terrains of the Iberian Chain and the sedimentary Tertiary terrains of the Pre-Pyrenees and the Ebro Valley. The lower part of the Ebro River flows across the Neogene Mora Depression, bends through the Catalan Chain and the Baix Ebre Valley, and at its lowest part flows across the delta plain deposits (Maldonado, 1972; Fig. 1).

Hydrological regime

The monthly discharge of the Ebro River is quite irregular. The northern tributaries provide a greater contribution to the Ebro River discharge than those in the south because the drainage basins of the former are located in areas of higher rainfall. In the lower part of the river (Tortosa station) the highest average monthly discharges are in winter and spring (600- 900 m3 s-') and the lowest discharges in summer ( ~ 2 0 0 m3 s - ' ; Maldonado, 1986).

Sediment dynamics in the lower Ebro River 569

. . . . 1 . .

Gracia I s l a n d

Sect ion m ( R i v e r Mouth)

Sect ion II (De l tebre)

I I

Migjorn River

1 Oo4Of 0°50' 0 4 krn I 1

Fig. 2. The lower part of the Ebro River, between the town of Amposta and the river mouth, showing the sampling stations.

During this century, the annual mean water dis- charge of the Ebro River has been very irregular (Fig. 3). The maximum mean water discharge was 992 m3 s - l in 1915, and the minimum was 165 m3 s - l in 1949. There is a general trend of decreasing mean annual discharge from 1914 to the present with some years of high discharge (1915 and 1960). There was a high discharge in 1937 when a strong flood led to a change in the location of the river mouth (Maldonado, 1972), but unfortunately there are no data for the late 1930s. The decrease in the annual water discharge

has been quite continuous from 1960 to the present. The 1980s were especially dry years: in 1982, 1986, 1987 and 1989 the mean discharge was lower than 250 m3 s- (Fig. 3).

River morphology

The morphology of the channel floor in the study area has been studied from sonographs and bathymetric profiles (DGPC, 1985; Guillen et al., 1989). The Ebro River enters the delta plain near the town of Amposta,

0 Annual sediment discharge I I I

0 1930 I950 I970 I990

Fig. 3. Annual water and sediment discharge of the Ebro River during the twentieth century (water discharge data from 'Confederacion Hidrografica del Ebro'; sediment discharge estimated from references given in Table 1).

570 J . GuiMn and A . Palanques

located 28 km upstream from the river mouth. The mean slope of the river in this stretch is very low (0.00038%; Davy, 1978) and the mean width of the river is 200 m. On the delta plain, the Ebro River has three major curves centred at 10, 17 and 23 km from the mouth. Two islands, Gracia Island and Sapiiia Island, are located between 15 and 18 km from the mouth. The mean depth of the river in the delta plain ranges from 4 to 6 m, although deeper (up to 21 m) and shallower (less than 3 m) areas are present.

Sonographs of the river bed show bedforms of different scales and morphologies. In some areas of the river between Amposta and Gracia Island, bedforms range from 0.2 to 0.9 m high, with wave- lengths from 8 to 30 m. The larger bedforms in that part of the river have linguoid crestlines. From Gracia Island to the river mouth, the size of the bedforms decreases (GuillCn et al., 1989).

F I E L D DATA

The lower part of the river has components of both marine and fluvial influence. In general, at high water discharge, the seawater-freshwater interface is located near the river mouth and only fluvial water circulates along the lower river channel. At low water discharge, a salt-wedge intrudes upstream below the river water.

Field data were taken systematically at three stations : Amposta, Deltebre and the river mouth (Fig. 2). The mean water depth at the Amposta and Deltebre stations is 5 m and at the river mouth station it is 6 m. Vertical profiles of salinity, current velocity and suspended particulate matter (SPM) concentra- tion, together with the mean grain size of bottom sediment, are shown in Fig. 4.

Without a salt-wedge intrusion

During the May 1988 survey period the water discharge was relatively high (675 m3 s- ')and no salt- wedge intrusion was detected in the lower part of the river. The current velocity was between 80 cm s-' at the surface,and 50 cm s- ' near the river bed. Vertical profiles of the SPM concentration were relatively constant at Amposta (15-18 mg 1- I ) and at Deltebre (12-16 mg I - ' ) , whereasattherivermouthstationthe near-bottom SPM concentrations were slightly higher than the surface concentrations (1 8-22 versus 15- 18 mg 1-*, respectively; Fig. 4a). The bottom sedi- ment was of sand grade at Amposta and Deltebre and of muddy sand grade at the river mouth.

With a salt-wedge intrusion

During the other survey periods the water discharge was considerably lower (1 10-230 m3 s- I ) than during the May 1988 survey period and a salt-wedge was present.

At the Amposta station the SPM concentration ranged between 5 and 31 mg I - ' , increasing with depth. The current velocity decreased from 17- 30 cm s- ' at the surface to 8-15 cm s- ' near the bed. The bottom sediment was of sand and muddy sand grade.

At Deltebre the SPM concentration ranged from 5 to 15 mg 1- ' in the freshwater and from 4 to 7 mg 1- ' in the salt-wedge, but in some surveys it increased to about 30rngI-l near the bottom and near the freshwater-saltwater interface. The flow velocity was about 2&30cms-' in the freshwater and was less than 7 cm s- ' in the salt-wedge. The bottom sediment was of mud and sandy mud grade.

At the river mouth station SPM concentrations ranged between 5 and 16 mg I - ' in the freshwater and around 4 mg I - ' in the saltwater. In some surveys, however, SPM concentration increased to about 20 mg 1-' near the river bed. The current velocity ranged between 26 and 80 cm s-' in the freshwater and was less than 7 cm s- in the salt-wedge. The bottom sediment was of mud grade.

In the summer surveys the temperature ranged between 24.5 and 27.0"C in the freshwater and between 22.0 and 25,O"C in the salt-wedge. The corresponding winter temperatures were between 9.3 and 12.8 and between 10.0and 14.1"C. The freshwater temperature was between 1.8 and 3.7"C higher than in the salt-wedge in summer and between 0.5 and 2.7"C lower in winter. The salinity of the fresh river water ranged from 0.55 to 0.70%, at Amposta and increased downstream, reaching 4.50%, at the river mouth station. The downstream increasing gradient of salinity in the freshwater is regulated by the salt- wedge intrusion. The more the salt-wedge intrudes upstream the stronger the salinity gradient is. The salinity inside the salt-wedge was higher than 35.0%,, except in its upstream limit where it was lower.

C H A R A C T E R OF T H E S A L T - W E D G E I N T R U S I O N

From the 'field data it is possible to distinguish four different situations with regard to the salt-wedge : (1) without salt-wedge intrusion, when only river

Sediment dynamics in the lower Ebro River 571

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Fig. 4. Vertical profiles of current velocity (+, m s '), salinity (A, %J, suspended particulate matter concentration (0, mg I ') and mean grain size of bottom sediment (0, units) recorded at the three sampling stations shown in Fig 2 (a) May 1988, water discharge=675 m3 s - ' , (b) September 1988, water discharge=230 m3 s - l , (c) January 1989, water discharge=210m3 s - l , (d) July 1989, water discharge=110m3 s - ' , (e) October 1989, water discharge=150m3 s - ' , ( f ) January 1990, waterdischarge= 165 m3 s - ' , (8) February 1990, waterdischarge=200 m3 s- ' Axisdetailsas In (a)throughout

572 J . GuiIIkn and A . Palanques

0

water flows along the lower part of the river; (2) the salt-wedge intrudes upstream from the river mouth but does not reach the Deltebre station; (3) the salt- wedge intrudes upstream from the Deltebre station as far as Gracia Island; and (4) the salt-wedge intrudes upstream beyond Gracia Island. The location of the salt-wedge in the study area is mainly regulated by the water discharge. With water discharge above 400 m3 s- ' the salt-wedge is pushed out of the river. In general, with decreasing water discharge the salt- wedge intrudes further upstream.

However, there are other factors controlling the position of the salt-wedge. One of these is the morphology of the river bed (the topographic high at Gracia Island, in particular, acts as a threshold for the salt-wedge intrusion). The upstream limit of the salt- wedge is generally located at this topographic high when the water discharge is below 350-400 m3 s-' and above 150-200 m3 s- ' . In this situation, the thickness of the salt-wedge is about 4 m at the river mouth and about 2 m at Deltebre. Usually, the upstream limit of the salt-wedge extends beyond Gracia Island when the water discharge decreases below 150-200 m3 s- ', and it can then reach the town of Amposta. In this situation the salt-wedge is about 4.5 m thick at the river mouth and 2.5 m thick at Deltebre. During the lowest stages of water discharge the salt-wedge may even intrude several kilometres upstream from Amposta (Aragon, 1943; Muiioz, 1990). When the water discharge increases after a period of low water discharge, the upper limit of the salt-wedge migrates downstream leaving salt water in the topographic lows of the river bed. Profiles of the typical salt-wedge intrusions are shown in Fig. 5.

In addition to the water discharge and the morphol- ogy of the river bed, the intrusion of the salt-wedge is also regulated by variations in sea level. GuillCn et al.

Amposto Gracia Is land Deltebre R i v e r Mouth

1 1 I I

(1990) observed that the study area is affected by a mixed microtidal regime with two asymmetrical cycles (one cycle of 15 cm and another of 7 cm), and they suggested that these cycles regulate the internal circulation flow of the salt-wedge (see next section) and produce upstream-downstream displacements at constant water discharge. Similar effects can be produced by the sea level changes induced by atmospheric pressure conditions and wind.

Usually, when the river water discharge decreases, the salt-wedge intrudes upstream and its thickness increases. However, under certain circumstances, such as during periods of low sea level and/or semi- blockage of the river mouth by sand bars, the salt- wedge can be moved downstream, and its thickness decreases even with a low water discharge. This situation was observed during the January 1990 and the February 1990 survey periods when, with a water discharge not higher than 200 m3 s- ' , the salt-wedge intruded only marginally upstream from the river mouth because the mean sea level was 15 cm below that recorded during the other survey periods.

CIRCULATION

The current velocity of the river water varies between 8 and 81 cm s-'. Maximum velocities were recorded at the Amposta station during the May 1988 survey period when there was a relatively high water discharge and maximum velocities were recorded at the River Mouth station during periods of low water discharge. When there was no salt-wedge, freshwater velocity increases when the water discharge increases and the mean velocity decreases from Amposta to the river mouth. However, when the salt-wedge is present, the velocity of the freshwater at the river mouth

Fig. 5. Idealized cross-section showing the river bed morphology and the different degrees of salt-wedge intrusion observed during the sampling period. Lines a, b and c represent the three typical situations of salt-wedge intrusion in the Ebro River.

Sediment dynamics in the lower Ebro River 573

40 3 5 -

o 3 0 - 2 2 5 - 21 20 ._ 15-

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station is much higher than at the Amposta station because of the decrease in the thickness of the freshwater flow.

The circulation in the salt-wedge is mainly regulated by the balance between sea level variations and the river water discharge. Current records taken for this study during periods of steady water discharge show that currents inside the salt-wedge are correlated with the mixed tidal cycles (Guilltn et al., 1990). During the October 1989 survey period the spring high tide produces an upstream flood mean flow of 9 cm s - ' and the neap high tide causes upstream flood mean velocities of 5 cm s- ' . The spring low tide generates a downstream ebb flow with a mean velocity of

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SPM mean gra in size ( p m )

9 cm s- ' . Still flow periods correspond to the neap low tide and to the periods between the falling and rising tides (Guillh et al., 1990).

SUSPENDED SEDIMENT

The SPM concentration varies between 5 and 32 mg 1-' in the upper freshwater layer, and between 3 and 31 mg 1-' in the salt-wedge. At most stations, the SPM concentration is lower in the salt-wedge (generally 3-8 mg 1 - I ) than in the freshwater (gener- ally 8-20 mg 1-' ; Fig. 6a). In some cases, however,

( f )

22 r

4 , s . 2 I I I I I I 1 0 10 20 30 40 50 60 70 80

SPM mean g r a i n s l z e (pn)

Fig. 6. Scatter plots of the data: I, samples taken near the freshwater-saltwater interface; NB, samples taken near the river bed; S.W., samples taken in the salt-wedge.

574 J . GuillPn and A . Palanques

the SPM concentration near the river bed is higher than in the surface freshwater. The average SPM concentration in the lower part of the river during the seven survey periods was about 15mgl- ' in the freshwater and 8 mg 1-' in the salt-wedge. In the river water, the average SPM concentration recorded at each survey decreases from 18.1 mg 1-' in May 1988 to 12.1 mg 1-' in February 1990.

The SPM concentration and the vertical SPM distribution in the lower part of the river are greatly influenced by the salt-wedge intrusion. When the river discharge is high enough to prevent the salt-wedge intruding upstream, the vertical SPM distribution is homogeneous (Fig. 4a), whereas when the salt-wedge intrusion is present, it is quite variable: the SPM concentration in the freshwater layer sometimes increases near the interface, decreases in the salt- wedge, but increases near the bottom at some stations (Fig. 4b,d,e). Increases in the freshwater SPM concen- tration near the interface can be related to the retention of settling particles over the discontinuity of density located between the saltwater and the fresh- water, and also to the physical processes that take place in the boundary layer between both water flows. The decrease in SPM concentration in the salt water is due to the lower SPM concentration of the marine water ( < 4 mg I - ' ; Palanques, 1987; Palanques & Drake, 1990), the low flow velocity inside the salt- wedge (Fig. 6c) and the rapid settling of the particles flocculated at the interface (Beck et al., 1974; Rashid, 1974; Monaco, 1977). The increase of the SPM concentration near the river bed may result from settling particles that remain in suspension when turbulence is produced in the sediment-water bound- ary layer. Near-bottom turbulence must increase when current velocity oscillations are produced in the salt- wedge. Biological reworking can also contribute to an increase in the SPM concentration near the river bed.

The SPM concentration does not show a clear trend with regard to the current velocity. A wide range of SPM concentration values are detected at different velocities, especially at low velocities (Fig. 6b). In fact, the lowest velocities are associated with both the highest and lowest SPM concentration values (Fig. 6b): The higher SPM concentration values, however, correspond to samples taken near the freshwater-saltwater interface and near the river bed which are most affected by boundary layer dynamics. If these samples are excluded from Fig. 6(b), a slight trend of SPM concentration increasing with flow velocity is apparent (Fig. 6b).

The mean grain size of the suspended particles in

the river freshwater is mainly between 10 and 30 pm, with only a few coarser samples (Fig. 6d). The mean grain size of the salt-wedge SPM is between 5 and 15 pm (Fig. 6d). The average mean grain size is 19.2pm in the freshwater and 9.9pm in the salt- wedge. The average SPM mean grain size in the salt- wedge is similar to that measured in the continental shelf water (8-12 pm; Palanques, 1987; Palanques et al., 1990a).

The SPM mean grain size shows no clear correlation with the current velocity. Low SPM mean grain size values are detected both under low and high current velocity (Fig. 6e), and although current velocity is higher in the freshwater, the mean grain size of the freshwater SPM is not always significantly higher than that of the salt-wedge SPM (Fig. 6d,e). However, the SPM concentration shows a reasonable correlation with the SPM mean grain size. Figure 6(f) shows a clear trend of SPM mean grain size increasing with SPM concentration.

BOTTOM SEDIMENT

The grain size of the bottom sediment in the lower part of the river is highly influenced by the salt-wedge dynamics. During the survey periods, when only freshwater circulated in this part of the river, the velocity near the river bed ranged from 7 to 50 cm s - ' and most of the bottom sediment was sand with a mean grain size of between 1 and 44 (Fig. 7). When the salt-wedge intruded into the lower part of the river, the near-bottom velocity was markedly reduced (Fig. 6c), and bottom sediment was mud and sandy mud with a mean size of between 5 and 74 (Fig. 7).

In fact, there is a gradation of the mean grain size as a result of the progressive accumulation of mud (about 7$) over the bottom sand with salt-wedge intrusion and as a result of the resuspension of this mud when the salt-wedge is not present. With salt- wedge intrusion, the mean grain size of the surface sediment becomes progressively finer with time, until after some weeks or even months of salt-wedge intrusion the surface layer of mud is thick enough to be sampled without being mixed with the underlying sand. The progressive decrease in the mean grain size of the surface sediment was observed at the Deltebre and river mouth stations during the period of permanent salt-wedge intrusion that took place from

Sediment dynamics in the lower Ebro River

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5 1

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0.. 8 0

0 0

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0 0

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Fig. 7. The mean grain size of the bottom sediment versus (a) river bed.

July 1988 to October 1989 (Fig.4a+). During this period the mean grain size of the bottom sediment decreased from 1.84 (Deltebre) and 3.34 (river mouth) in May 1988 without salt-wedge intrusion, to 74 and 6.54 respectively in July and October 1989 (Fig. 4).

When the salt-wedge is pushed downstream, only freshwater circulates and the flow velocity near the bottom increases. When the flow velocity becomes high enough, the bottom mud deposited under steady salt-wedge conditions is resuspended and transported downstream, and the bottom sediment becomes sand again (Fig. 7b), except in the pools and depressions where mud remains permanently.

The mean grain size of the sampled bottom sediment is thus related to both the flow velocity and the period of time under salt-wedge intrusion. During the May 1988 survey period, flow velocity near the bottom was quite high (about 50 cm s - ') and the bottom sediment was sand with a mean grain size of between 1.2 and 3.741, increasing towards the river mouth station (Fig. 4a). The data recorded at the Amposta station during the July 1989 survey period and at the Deltebre station in the January 1990 survey period show a situation in which the head of the salt-wedge is located nearby (a small increase of salinity is detected near the bottom in both stations) and the bottom sediment is in the transition zone between the influence of the salt-wedge and the freshwater. In these two examples the flow velocity near the bottom was still very low and bottom sediment had a mean size of about 4-54 (Fig. 4d,f,g). During the following surveys the flow velocity increased sufficiently at the Amposta station to resuspend the surface mud layer, thereby increasing the mean grain size of the bottom sediment, but it did not increase sufficiently at the Deltebre station, where the mean grain size remained the same as during the previous sampling interval (Fig. 4e,f,g).

Veloci ty ( c m s-')

' salinity 1 m above the river bed and (b) velocity 1 m above the

ESTIMATION OF THE AMOUNT OF SUSPENDED SEDIMENT

TRANSPORTED BY THE RIVER DURING THE LAST CENTURY

The suspended sediment load transported by rivers is controlled by several factors defined in Milliman (1980) and Berner & Berner (1987). In the Ebro River, some of these factors favour a high concentration of suspended sediment. First, most of the northern tributaries rise in the Pyrenees, where relief is high and stream discharge increases when the winter snow melts. Second, the river flows over large deforested arid and semi-arid areas covered by sedimentary deposits, some of which are easily eroded. Third, several areas of its drainage basin are sporadically affected by quite high rainfall.

The suspended sediment concentration in rivers with similarities to the Ebro, such as some California rivers, ranges from 290 to 3000 mg 1-' (Janda & Nolan, 1979). According to discharge curves of these rivers, the Ebro River sediment discharge should be about 20 x lo6 tons per year (Palanques et al., 1990a). Accepting a mean annual water discharge of about 1.9 x 10" m3 for the Ebro River, the average sus- pended sediment concentration should be about 1000 mg 1- '. However, the suspended sediment con- centration detected in the lower course of the river at present is much lower. This is due to the high sediment retention caused by the dams constructed along the Ebro River and its tributaries. Data given by Bayerri (1934-1935) before the constructionof the major dams suggest that suspended sediment concentration in the river water was 770-1700 mg 1-' and that it may have increased up to 10 g 1-' during maximum river flow. It is therefore concluded that the suspended sediment concentration before construction of the dams was as

576 J . Guillkn and A . Palanques

Table 1. Details of the main dams located in the Ebro River drainage basin, along with historical data of suspended sediment discharge of the Ebro River during the last century.

Year Dam (hm3) ( x lo6 tons per year) Source Capacity Sediment discharge

1880 1906- 1930 1911 1934 1940-1950 1946 1959 1962 1966 1968 1969 1983-1986 1988-1990

- - 25 Gorria (1880)*

- - 17 Carreras Candi (1911)* - - 14 Bayerri (1934-1935)

several 0.3-200

several 50-200 Ebro 540 Yesa 470

Mequinenza 1000 Mediano 436 Ribarroja 136

- - 2.2 Catalan (1969)

- - 0.15 Palanques (1987) - - 0.12 this DaDer

* Cited in Arredondo (1982).

high as that deduced from the data of Bayerri (1934- 1935).

Historical data reveal the trend of the annual suspended sediment discharge from 1880 to the present day. Table 1 shows the decreasing annual sediment discharge related to dam construction during the last century. This decrease is very sharp after construction of the first large dams (Ebro and Yesa; Fig. 1) during the 1940s and 1950s. Construction of the largest dams (Mequinenza and Ribarroja) in the lower part of the river during the 1960s further reduced the sediment discharge.

Data taken during this study give an estimate of the sediment discharge of about 0.12 x lo6 tons per year. This is less than 1% of the suspended sediment discharged by the river before construction of the dams, as has been reported in previous studies (Palanques & Drake, 1987; Palanques et al., 1990a,b; Fig. 3, Table 1). Although present estimates have been made mainly from data relating to relatively dry years, sediment discharge does not exceed about 5% of the pre-dam sediment discharge during wet years (Pal- anques et al., 1990a,b).

DISCUSSION

The hydrodynamics and sediment dynamics of the Ebro River have changed during the last century. The increase in human management of the river water has contributed to the decreasing trend in the annual mean water discharge (Fig. 3). At present, the water discharge in the lower part of the river is regulated by the last downstream dam, which is required to deliver

a minimum discharge of about 150 m3 s - ' for the necessities of a nuclear power station located a few kilometres downstream. Near the delta, up to 50 m3 s-' of water can be subtracted from the river when the two main irrigation channels of the delta are opened (Camp & Delgado, 1987). Therefore, a water discharge of about 10tL200 m3 s-' is normally main- tained in the lower part of the river during dry periods, as has been observed in this study (Fig. 4). This regulation of the water discharge contributes to the stable position of the salt-wedge during dry periods. Prior to dam construction, water discharge was far more variable and the salt-wedge may have intruded further upstream than is seen today.

The estuarine circulation in the lower part of the Ebro River is equivalent to the type 'A' circulation defined by Pritchard (1955). The Ebro River estuary has a particularly small tidal range ( < 20 cm) and has a distinctive morphology. In contrast to other river estuaries, it is narrow (about 200 m) and shallow (5- 6 m) and its width and depth are relatively constant along several tens of kilometres. When the salt-wedge is present, the estuarine circulation is always well stratified with a very strong gradient between the freshwater and saltwater layers (Fig. 4). The salt- wedge also influences transport of the sediment load. In the salt-wedge current velocities are below 15 cm spl. Under these conditions there is no bedload transport and part of the SPM is deposited on the channel.

Human management of the river during the last century has also drastically reduced the sediment discharge. The largest contribution to this decrease has resulted from construction of dams. Dams retain

Sediment dynamics in the lower Ebro River 577

most of the solid load transported by the river and mitigate the large floods during which huge amounts of sediment were transported under pre-dam condi- tions. The sediment retention due to such structures depends both on the number of dams and on their location (Palanques et al., 1990a,b). Dams located in the upper course of the river and tributaries, where erosive processes are intense, should have a significant impact on retaining sediment. Dams built in the lower part of the river retain the sediment load from a larger portion of the drainage basin and therefore trap a higher proportion of the sediment load that would be discharged into the sea.

Several dams have been built along the tributaries of the Ebro River (Fig. 1). These dams cause an accumulative sediment retention and the sediment load transported to the dams located in the lower course of the river (Mequinenza and Ribarroja) is already very low (about 0.5 x lo6 tons per year; Palanques, 1987; Palanques et al., 1990a). Figure 3 and Table 1 show that the decrease in river sediment discharge was already significant before the Mequi- nenza and Ribarroja dams had been constructed. The Mequinenza and Ribarroja dams have been estimated to retain at least 75% of the sediment load (Palanques et al., 1990).

In the Ebro River, the only sediment that could be transported without suffering any kind of retention is that eroded from the channel floor downstream of the dams or supplied by the small tributaries that merge with the river below the last dams. At present, the output of suspended sediment from the Mequinenza and Ribarroja dams has been estimated to be about 0 . 1 2 ~ lo6 tons per year (Palanques et al., 1990a), which is similar to the amount of sediment discharged by the river into the sea. Most of the sediment discharged by the Ebro River is therefore the fine sediment not trapped by the dams, and inputs from channel floor erosion and from the small tributaries merging downstream of the last dam are insignificant to the mean annual discharge during the study period.

The concentration and mean grain size of the SPM leaving the dams is inversely proportional to the residence time in the reservoir. During periods of low rainfall and low fluvial discharge, the residence time of the SPM in the dams increases, and thus the average SPM concentration in the river water decreases, as was detected between the May 1988 and February 1990 survey periods. When there is a higher water discharge released from the dams and the current velocity increases in the lower part of the river, there is no increase in the mean grain size of the SPM

transported downstream because there is no coarser particulate matter suspended in the reservoirs. This explains why the SPM mean grain size is reasonably correlated with the SPM concentration (Fig. 6f) but not with the current velocity (Fig. 6e).

The bedload transported by the river is also retained by the dams. In the lower part of the river, the bedload transport was very low and bedforms were inactive and covered by mud during the study period (GuillCn et al., 1992). Thus, the Ebro delta coast receives practically no sand sediment from the river and therefore marine processes dominate the present evolution of the delta (Maldonado, 1986). Recent studies have reported a sediment deficit on the Ebro delta coast (JimCnez & Garcia, 1991). This shortage of sediment is a direct result of the decrease of that supplied by the Ebro River.

On the Ebro continental shelf there is a prodelta mud belt created by the rapid accumulation of fine sediment discharged by the Ebro River during the Holocene (Diaz et al., 1990). This discharge should have generated an associated plume of high SPM concentration. However, in the present river plume, SPM concentrations are relatively low (0.6-4 mg 1- ') and resuspended SPM dominates over new SPM discharged directly from the river (Palanques et al., 1990a,b), as a consequence of the shortage of fluvial sediment input.

Damming is known to affect many other fluvial sedimentation systems. The decrease in sediment discharge produced by the trapping effect of the dams is variable. For example, whereas the Mississippi River discharge has been reduced by about 30% as a result of dam construction, in the Nile River construc- tion of the Aswan dam has reduced the sediment discharge by about 95% (Milliman & Meade, 1983; Shahin, 1985). The situation in the Nile is more similar to that in the Ebro because both have dams built near their mouths and in both most of the sediment load (> 95%) is retained by the dams.

CONCLUSIONS

The lower part of the Ebro River is a microtidal salt- wedge estuary. Intrusion of the salt-wedge upstream is mainly regulated by the water discharge and the morphology of the river bed and the river mouth. Although the system is microtidal, sea level variations caused by tides and atmospheric conditions (wind, atmospheric pressures) also influence the dynamics of the salt-wedge intrusion. Circulation inside the salt-

578 J . GuillPn and A . Palanques

wedge is thus regulated by the balance between variations in sea level and the river discharge.

The SPM concentration and the vertical SPM distribution in the river are greatly influenced by the salt-wedge intrusion. The average SPM concentration in the lower part of the river during the sampling period was about 15 mgl- ’ in the freshwater and 8 mg 1- ’ in the salt-wedge, which are very low for a river such as the Ebro.

At present the annual suspended sediment discharge is lower than 2 x lo5 tons, less than 1% of the annual suspended sediment discharge in the late nineteenth century. This huge reduction in the sediment input is mainly the result of sediment retention behind the dams built along the Ebro River and its tributaries.

Low annual rainfall during several of the last few years and the increase in human management of the river water have also contributed to a decrease in the annual river water discharge, and therefore to a n even greater decrease in the amount of sediment discharge. The annual mean water discharge has been especially low during the late 1980s. This led to the intrusion of the salt-wedge for long periods of time. Although the river sediment input was very restricted, these long periods of salt-wedge intrusion have allowed a significant layer of fine mud to accumulate. This layer is resuspended and eroded when the salt-wedge is pushed downstream by increased river flow. The remaining sediment is thus the underlying sandy sediment that cannot be transported under the present hydrodynamic conditions.

At present, the Ebro River transports a very reduced suspended sediment load to the marine environment and no significant bedload transport has been detected. Thus, the Ebro delta receives practically no new sandy sediment from the river and supplies of fine sediment to the continental shelf have been drastically reduced.

ACKNOWLEDGMENTS

We thank Jordi Camp, Ignacio Diaz and Marcelli Farran for advice, scientific discussion and assistance in the surveys. We also thank D r A n d r b Maldonado for his support. This manuscript benefited from critical reviews by Bruce S. Hart and Hans Nelson. Jose M. Anguita and Neus Maestro are acknowledged for their technical support. Teresa Moya and Josep SapiAa are also gratefully acknowledged. This work has been supported by the ‘Direcci6 General de Ports i Costas de la Generalitat de Calalunya (Autonomous Govern-

ment)’ under a collaboration agreement between the ‘Consejo Superior de Investigaciones Cientificas’ (Marine Science Institute) and the ‘Universitat Poli- tecnica de Catalunya’ (Marine Engineering Labora- tory).

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Sediment dynamics in the lower Ebro River 579

(Manuscript received I0 January 1991 ; revision received 13 March 1992)