Late Pleistocene–Holocene hydrologic changes in the interfluve areas of the central Ganga Plain,...

Late Pleistocene–Holocene hydrologic changes in the interfluve

areas of the central Ganga Plain, India

Pradeep Srivastavaa,1, Indra Bir Singhb, Shikha Sharmab, Uma Kant Shuklac,Ashok Kumar Singhvia,*

aPlanetary and Geoscience Division, Physical Research Laboratory, Navrangpura, Ahmedabad-380 009, IndiabDepartment of Geology, Lucknow University, Lucknow-226007, IndiacDepartment of Geology, Kumaun University, Nainital-263 002, India

Received 11 October 2000; received in revised form 4 November 2002; accepted 5 December 2002

Abstract

Abandoned channel belts, ponds and point bar deposits of palaeochannels in the interfluve regions of the central Ganga Plain

suggest changes in the morphohydrologic conditions during the Latest Pleistocene–Holocene. Stratigraphy of these ponds

comprises channel sand at the base overlain by shell-bearing clayey silt. The contact of the two facies marks the phase when

channels converted into standing water bodies. Point bar deposits of some palaeochannels are overlain by oxidised aeolian sand,

indicating that the channel abandonment possibly occurred due to the desiccation and aridity in the region.

Optically stimulated luminescence (OSL) chronometry of the pond sediments suggests that the deposition of the basal

channel sand started before 13 ka and continued up to f 8 ka. The ponds formed around 8–6 ka when the channel activity

ceased. Evidence from the point bar deposits also indicates that the fluvial activity in the region ended sometime during 7–5 ka.

This was followed by aeolian aggradation. The present study thus suggests that the hydrologic conditions in the Gangetic plains,

i.e. initiation of channels and their abandonment, formation of microgeomorphologic features such as ponds and their eventual

siltation, were controlled largely by climatic changes (i.e. monsoon changes) supported by tectonic activity. For the past 2 ka,

increasing human and related agricultural activity has substantially accentuated the natural siltation rate of ponds.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Ganga Plain; Interfluve; Ponds; Alluvial ridges; Luminescence dating; Holocene climate change

1. Introduction

Abandoned channel belts, ponds and palaeo point

bar deposits have been useful in palaeohydrologic

reconstructions. Palaeo river changes in response to

climate changes have been investigated in detail in

South East Asia, Australia, Africa and South America

(Bishop and Godley, 1984; Rotnicki, 1991; Thomas,

2000). However, in the Indian context, such studies

have been minimal.

The Ganga Plain in northern India is a major

component of the Himalayan foreland basin and is

one of the largest, fluvially controlled depositional

systems of the world. This region has three distinc-

0169-555X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0169-555X(02)00361-6

* Corresponding author. Tel.: +91-79-6302129x4366; fax: +91-

79-6301502.

E-mail address: [email protected] (A.K. Singhvi).1 Present address: Department of Geography, University of

Georgia, Athens, GA 30602, USA.

www.elsevier.com/locate/geomorph

Geomorphology 54 (2003) 279–292

tive geomorphic surfaces of regional extent. It has

been suggested that these surfaces formed as a

consequence of climatic change and associated base

level conditions during the Late Pleistocene–Holo-

cene (Singh, 1996).

The Upland Interfluve Surface is the most prom-

inent and widespread geomorphic surface of the

Ganga Plain. In the present study, geomorphic and

sedimentological analysis of the sediments of aban-

doned channel belts, associated point bar deposits

(alluvial ridges), ponds and lakes on this surface along

with their luminescence dating were used to recon-

struct palaeohydrologic changes through time.

2. Geomorphic setting

The study area spans part of rivers Ghaghara–

Gomati, Gomati–Sai and Sai–Ganga interfluves

(Figs. 1 and 2). Satellite data (Fig. 1a) show that,

(i) the river Ganga (SW corner of the imagery) is

strictly braided and has f 12 km broad flood plain

with meander scars on its river valley terrace (T1

terrace) and, (ii) the Sai and the Gomati Rivers

(extreme NE corner) are meandering streams with

narrower (f 1 km) flood plains. Regional slope of

the area is NW–SE. A cross-section along A–AVshows that River Ganga is incised by f 25 m into

the upland terrace (T2 terrace), the Gomati up to 10

m and the Sai up to f 7 m (Fig. 1b). The region

between the two river valleys occurs as a raised 20–

50 km broad interfluve area on which smaller sub-

watersheds (up to fourth order), abandoned channels

belts and alluvial ridges occur. The inference that

interfluves (T2 terrrace) are the oldest geomorphic

surfaces is based on the fact that all the other

surfaces are either superimposed or incised into it.

Earlier luminescence ages from near the top of this

surface in the region suggest that its aggradation

continued at least until 7 ka (Srivastava et al.,

2003a). The sedimentary sequence of this surface

comprises fine sand and silt with calcrete layers,

suggesting its formation due to the activation and

abandonment of smaller river channels, lake activity

and sheet flows on flat surfaces (Singh et al., 1999).

Presently, f 30% of the interfluve area is being

drained by small, highly sinuous, underfit channels

that are incapable of transporting any appreciable

quantities of sediment load. Locally referred to as

Tals, ponds are small, closed water bodies that act as

sediment sinks for the rest of the area. Alluvial

ridges occur as sandy mounds within abandoned

channel belts and often preserve the point bar depos-

its of the paleochannels (Pc).

In the present study (spanning f 4000 km2 area),

three representative ponds and two alluvial ridges

from Gomati–Sai and Sai–Ganga River interfluves

were examined and based on their sedimentary suc-

cessions, luminescence chronology and the regional/

global climatic record, palaeohydrologic changes in

the region are inferred.

3. Morphology and sedimentology of ponds

The dimensions of the ponds are variable and their

depth ranges between 1 and 5 m. They are local sinks,

where sediment–water transfer into the pond occurs

by sheet flow and minor rills. No delta building takes

place and no outflow channels that could drain out the

pond water, exist. The water budget of the ponds

shows seasonality such that during the summer

months some of the ponds dry out.

Based on an analysis of shapes and sizes of 1388

ponds, three basic pond types were identified (Figs. 2

and 3):

1. Type 1 ponds have an area of >0.1–2.7 km2. These

ponds are perennial and occur as chains of arcuate

and linear lakes associated with linear belts of

abandoned channels and their cutoff meanders.

Occasionally, circular or subcircular shapes also

occur. About 14% of the ponds accounting for

f 50% of the total ponded area belong to this

category.

2. Type 2 ponds occur as small and arcuate, linear or

oval-shaped water bodies and, have an area of

f 0.05–0.1 km2. About 7% of the ponds occupy-

ing >20% of the total ponded area belong to this

category.

3. Type 3 ponds occur in clusters, at times scattered

throughout the area and are generally located in

linear depressions. Such ponds have an area of

< 0.05 km2 and f 79% of the ponds covering

f 30% of total ponded area belong to this group.

P. Srivastava et al. / Geomorphology 54 (2003) 279–292280

Pond density, i.e. (total number of ponds/square

kilometre) varies from 0.73 to 0.1 and higher pond

density is accompanied by clustered occurrences of

ponds of Type 3 Stratigraphy of the larger ponds

comprises a sandy unit at the base overlain by lensoid

shell layer (marl deposit), followed by f 2–3 m

Fig. 1. (A) Satellite data showing the general geomorphology of the area. Note the occurrence of ponds as parts of paleochannels (Pc) in

interfluve areas (T2). T1 is a river valley terrace showing a meander scroll plain (SP). T0 is an active flood plain. (B) Cross-section along A–AV.Note the channel incision and raised interfluve areas and position of ponds and alluvial ridges.

P. Srivastava et al. / Geomorphology 54 (2003) 279–292 281

thick clayey silty sediment. The shell layer marks the

initiation of pond activity and has been radiocarbon

dated to 9–8 ka BP (Agarwal et al., 1992; Singh,

1996). In the present study, the ponds with clear

geomorphic evidences of an association with aban-

doned channel belt (sinuous and linear geometry)

were selected for sedimentological study and are

discussed below (Fig. 1).

Chunauti Tal: It is a linear pond f 125 m wide

and f 1000 m long. The basal unit comprises mica-

ceous fine sand grading into a 1.7-m-thick greyish

yellow silty fine sand unit. The sandy unit is capped

Fig. 2. Geomorphology of the study area. Water bodies viz, Chunauti Tal, Uganna Tal and Misa Tal, marked A, B and C, respectively, are

studied in detail for the stratigraphy and chronology.


by 0.75-m-thick black ferrugenised clayey silt with

sporadic gastropod and bivalve shells.

Uganna Tal: It is an arcuate pond inscribing an

island, has a length of f 1200 m and width of f 150

m. In a 1-m deep trench, the basal 30 cm is micaceous

fine sand (3–4 B), overlain by f 25 cm bioturbated,

ferrugenised silty clay. This is capped by 45-cm- thick

black clayey silt.

Misa Tal: It is an arcuate pond with a width of

f 150 m and a length of f 1500 m with a strati-

graphic sequence comprising basal micaceous fine

sand overlain by f 60 cm unit of shell-bearing

clayey silt. This silt is overlain by 1.8-m-thick black

clayey silt with gastropod shells. The clay percentage

varies up to 30%.

Overall, the pond sequences comprise grey col-

oured micaceous sands at the base followed by

lenticular units of shell and clayey sediments. The

facies association of sand overlain by shell-bearing

clayey silt suggests that an active channel carrying

fine sand was abandoned and converted into ponds

in which sedimentation of a clay-rich shell-bearing

unit occurred. The sediment type, the depositional

environment of pond-fill deposits and their geo-

morphic pattern (arcuate, linear) indicates their ori-

gin as a consequence of channel abandonment in

the region. The presence of bioturbated ferruginized

silty clayey units indicates sedimentation under

stagnant water conditions.

Table 1

Lithofacies of alluvial ridges

Lithofacies Thickness (m) Textural attributes

Cross-bedded

sand

0.5–1.5 grey, large-scale trough and

planar cross-bedded,

moderate mottling,

occasional clay balls

Parallel

laminated

sand

0.5–2.5 grey to yellowish grey,

individual laminae 0.5–1.0

cm thick, moderately mottled

Rippled

fine sand

0.2–0.3 grey to yellowish grey,

present in form of lenses in

different facies; ripple

wavelength V 20.0 cm and

height up to 5.0 cm

Bioturbated

fine sand

1.0–3.0 grey-buff, grey, extensive

mottling; both animal and

plant burrows are visible

Well-sorted

ferruginized

sand

1.0–3.0 yellow reddish yellow,

highly ferruginized,

no physical structures but

occasional burrows are visible

Fig. 3. Pond types in the region. The dotted line represents the depressed areas connecting two smaller ponds.


4. Alluvial ridges

Alluvial ridges are another important features

associated with the abandoned channel belts. Facies,

palaeohydrology and luminescence chronology of

these features have been reported by Singh et al.

(1998) and Srivastava et al. (2000). Morphology,

sedimentology and chronology of these features are

described below.

The ridges exist on the upland interfluve surface

and are of oval, arcuate and linear shaped with a

positive relief of 2–5 m and can be traced up to 0.25–

2.5 km. Table 1 and Fig. 4 summarise the lithological

details on the alluvial ridges. Overall, the stratigraphy

shows a fining upwards point bar sequence at the base

overlain by well-sorted, oxidised aeolian sand. Based

on the geometry of the sand body and the grain-size

changes in a similar point bar deposit (Gangaganj

ridge, Fig. 4), Singh et al. (1998) inferred that a

gradual decline of water discharge (from 447 to 7

m3/s) occurred with eventual abandonment of the

channel. Presence of wind blown sand over point

bar deposits suggests a period of enhanced aridity. A

similar inference has been made in the western Ganga

Fig. 4. Lithological succession and chronology of alluvial ridges (a) at Gangaganj and (b) at Gahira bypass.


Fig. 5. Palaeowatershed. Probable water divide demarcated on the basis of pattern of slope marked with thick arrows.


Plain. Slope and geomorphic analysis of the Survey of

India toposheets (1:50,000 scale) indicates a palae-

owatershed with 55 alluvial ridges within the palaeo

water-divide (Fig. 5).

5. Luminescence dating

Luminescence dating of sediments relies on the fact

that, during their predepositional weathering and trans-

port, daylight exposure of minerals constituting the

sediments bleaches their geological luminescence to a

near zero residual level (I0). Thus, at the time of their

sedimentation, the grains have zero or near zero

luminescence. On burial, further daylight exposure

ceases and reaccumulation of the luminescence signal

starts due to radiation exposure arising from the decay

of natural radioactivity viz. 232Th, 238U and 40K in the

sediment. A small contribution additionally comes

from the cosmic rays. Thus, reaccumulation of the

luminescence signal continues until the excavation and

sample collection. The total luminescence signal (Inat)

comprises the initial residual level I0 and the lumines-

cence (Id) acquired since burial (i.e. Inat = Id + I0). Id can

be related to the age of burial via the annual rate of

luminescence induction by the relation,

Age ¼ Paleodose=Annual dose ¼ P=DT

Where P is the equivalent dose or the paleodose

representing the laboratory radiation dose that induces

luminescence intensity Id. DT represents the total

annual radiation dose and is computed using the

elemental concentrations of natural radioactivity. The

event dated by the luminescence technique is the most

recent transport and burial event (see e.g. Aitken,

1985, 1998).

Aeolian sediments are transported by saltation and/

or suspension over long distances and consequently

experience direct daylight exposure for an extended

duration. This enables a reasonable assumption that

their geological luminescence is bleached to an opti-

mum residual level. Contrastingly, fluvially trans-

ported sediments experience a wide variety of day-

light exposure before deposition. This is due to

attenuation and modification of daylight and its spec-

trum, by the water column, turbulence, the grain size

and the sediment load. Thus, in general, the daylight

bleaching of fluvially transported sediments is hetero-

geneous and partial. The advent of Optically Stimu-

lated Luminescence (OSL) dating technique with

rapid bleachability of OSL signal helped in minimis-

ing some of these problems. The feasibility of using

infrared stimulated luminescence dating (IRSL) in

Ganga Plain was first demonstrated by Someshwarrao

et al. (1997) and Srivastava et al. (2003b) have app-

lied these methods in the region.

In the present study, samples from the basal chan-

nel sand and overlying clayey units were dated to

develop a chronology of the events of channel activ-

ity, their abandonment and the development of the

ponds. Sample nos. CH 1–3 were collected from

Chunauti Tal sequence, GP 20–21 from Uganna Tal

and ME 1–2 from Misa Tal. Fig. 1 provides the

sample locations and stratigraphic logs.

All the samples were pretreated with 10% HCl and

30% H2O2 to remove carbonates and organic matter.

The samples were then dry sieved to obtain the 105–

150 Am grain size fraction. Density separation using

Na-polytungstate (q = 2.58 g/cm3) was carried out to

separate Quartz and K-feldspar fractions. The Quartz

fraction was further etched with 40% HF for 80 min

followed by 12N HCl for 30 min to remove the alpha

skin. The separated mineral grains were mounted as a

monolayer on stainless steel discs using Silkospray.

For silt-rich sediments, the analysis was carried out on

4–11 Am polyminerallic samples extracted through a

sequential pretreatment of the sample with 1N HCl,

30% H2O2 followed by deflocculation using 0.01N

Na-oxalate and eventual extraction of the desired

grain size using Stokes settling times appropriate for

a 6-cm column of acetone. The extracted fine silt

fraction was resuspended in acetone and equal vol-

umes were pipetted onto aluminium discs kept in

small glass cylinders with acetone, and finally dried

at 50 jC in the dark.

Blue-Green light Stimulated Luminescence (BGSL)

and Infrared light stimulated luminescence (IRSL)

measurements were carried out on a Riso TL-DA-15

reader. IR stimulation used TEMT-484 infrared diode

arrays and a filtered halogen lamp was used for BGSL.

For IRSL, the detection optics comprised Corning 7-59

and Schott BG-39 filters coupled to an EMI 9635 QA

photomultiplier tube, whereas for BGSL, the detection

optics channel comprised two Hoya U-340 and Schott

BG-39 filters. Beta irradiation was made using a 25-


mCi 90Sr/90Y beta source. The paleodose (P) was

estimated using the additive dose method and a late

light subtraction was carried out. The dose rate estima-

tion relied on thick source ZnS (Ag) alpha counting

technique for elemental concentration of Uranium,

Thorium and NaI (TI) gamma ray spectrometry for

Potassium. Radioactive equilibrium was assumed in

the decay series. Though this assumption may not

always hold true for fluvial sediments, nearly 50%

contribution to the dose by potassium provides some

cushion against gross errors in the ages and correspond-

ing interpretations based on this assumption. Quartz

BGSL ages on sample no. CH1, CH2, GP20 and ME1

were determined after elevated temperature IR cleaning

at 220 jC for 5 min (Jain and Singhvi, 2001) with the

assumption that IR cleaning does not affect the Quartz

BGSL. Ages on samples CH3, GP21 and ME2 were

estimated using IRSL analysis of fine-grain polyminer-

allic aliquots. The choice of a fine-grain or coarse-grain

analysis was dictated by the mean grain size of the

sediment.

6. Results and discussion

Table 2 provides data on radioactivity, paleodose,

dose rate and the ages. In the following, the impor-

tance of these ages is discussed. Fig. 6 shows the

typical IRSL shine down curve, paleodose plateau and

growth curve for the sample GP-21.

6.1. Evolution chronology of ponds

Chunauti Tal: Two dates on the sandy, fining

upward sequence do not indicate depositional hiatus.

Basal age of the sandy unit was 11F 2 ka and the top

of this unit was dated to 8F 3 ka. The base of the

overlying muddy unit was dated to 7F 1 ka. These

ages suggest that the channel activity was established

prior to 11 ka and continued until 8 ka and the channel

abandonment occurred some time during f 8–7 ka.

Uganna Tal: The channel sand gave an age of

13F 3 ka, showing that this channel was initiated

before 13 ka. The age of the initiation of the muddy

unit was 7F 2 ka, indicating that the abandonment of

the channel and the formation of this pond occurred

some time around 7 ka.

Misa Tal: The sandy unit of this pond was saturated

with water, which precluded sampling. The age from

the muddy unit from just above the sand was 6F 1 ka,

indicating that here, the channel abandonment had

occurred prior to 6 ka.

The chronostratigraphic data suggest that the chan-

nel activity in the region began before 13 ka and

continued at least up to 8 ka. These channels were

abandoned some time during 8–6 ka. Point bar

deposits of paleochannels, preserved as alluvial

ridges, are overlain by brownish-coloured, well-sorted

aeolian sand in the region, which also suggests a

phase of channel abandonment during 7–5 ka (Fig.

4). This means that during z 13 ka, there was a phase

when many smaller channels, capable of carrying very

fine sand (3–4B) developed on the upland interfluve

terrace. Such small channels remained active up to 8

ka and thereafter; some of the channels became

abandoned, giving rise to the ponds. Some of the

larger channels, now seen as abandoned channel belts,

were active between >13 and 8 ka and became

abandoned around 8 ka forming large linear lakes.

While channels were once operating simultaneously,

their size and order should have determined the exact

timing of their abandonment. Occurrence of aeolian

Table 2

Radioactivity, paleodose, dose rate and ages obtained from sequences of ponds

Sample

no.

Depth

(m)

Mineral Technique U (ppm) Th (ppm) K (%) Dose rate

(Gy/ka)

P (Gy) Age (ka)

CH-1 2.70 quartz BGSL 5.4F 0.3 19.6F 6.7 2.1F 0.1 4.2F 0.5 45.6F 5.6 11.0F 2.0

CH-2 0.75 quartz BGSL 3.9F 0.8 21.2F 6.5 2.1F 0.1 4.3F 0.6 35.1F12.4 8.0F 3.0

CH-3 0.35 polyminerallic IRSL 4.3F 0.9 21.4F 7.7 2.1F 0.1 5.9F 1.1 40.3F 2.5 7.0F 1.0

GP-20 1.0 quartz BGSL 2.5F 0.9 6.5F 2.9 1.8F 0.1 2.1F 0.4 26.8F 2.8 13.0F 3.0

GP-21 0.5 polyminerallic IRSL 13.7F 1 13.8F 7.8 1.8F 0.1 6.1F1.2 41.9F 6.4 7.0F 2.0

ME-1 2.1 quartz BGSL 8.5F 0.4 13.0F 6.9 1.3F 0.1 5.0F 0.7 30.1F1.9 6.0F 1.0

ME-2 1.5 polyminerallic IRSL 8.9F 1.4 19.1F 8.6 3.3F 0.2 6.2F 0.8 14.3F 2.2 2.0F 0.5


sand in the region also suggests a phase of decreased

water budget and a more arid climate leading to

channel desiccation. Table 3 provides a compilation

of chronologies of channel abandonment in the area.

The luminescence dates are comparable with the

calibrated 14C-dates on shells given by Singh (1996)

and Agarwal et al. (1992), which probably shows that

the net effect of diagenetic alteration and reservoir age

is minimal and the radiocarbon ages, at least in such

cases are acceptable (Timothy, 1986).

The distribution pattern of the ponds and their

sedimentary sequence shows that they evolved due

to the abandonment. Analogous evidence of pond

formation in the central Ganga Plain has been

provided by Agarwal et al. (1992) and Singh

(1996). Although the size and the order of the

abandoned channels determined the initial shape

and size of ponds, continuing siltation and erosion

of individual catchments have since modified their

dimensions. Thus, clustered ponds in a linear depres-

sion are most likely to be due to subsequent siltation

leading to the fragmentation of the larger ponds and

the formation of smaller water bodies. Similarly,

varying pond densities in different regions may

reflect variation in palaeo drainage density, but this

aspect needs a confirmation.

6.2. Geomorphic evolution

In the following, an attempt is made to understand

the formation of ponds, alluvial ridges and the drain-

age system of interfluve areas of the central Ganga

Fig. 6. Typical IRSL (A) IRSL output with stimulation time. N refers to signal from sample as received and the other curves. N+ nGy refer to

samples additionally irradiated in the laboratory with calibrated beta source, (B) growth of luminescence with dose and (C) paleodose vs.

stimulation time.


Plain and their response to Late Pleistocene–Holo-

cene climatic change and other processes. Subwater-

sheds in the region, originating within the alluvial

plain, are usually of fourth order, 1–3 m deep and

< 30–50 m wide and are supported by precipitation

and ground water. Water in the Ganga basin is

provided by precipitation and glacial melt. Discharge

of Ganga River at Hardwar has f 15% contribution

from glacial melt and the rest is contributed by

precipitation (Das Gupta, 1975). The winter rainfall

in the region accounts for f 10% of the total rainfall.

This implies that the groundwater table in the Ganga

Plain depends largely upon the summer monsoon

rainfall. A drop in the water table consequently leads

to drying of dependent shallow channels. Thus, during

8–6 ka, when channel abandonment occurred and

formation of ponds began, a lowering of groundwater

table due to decreased rainfall and glacial melting

should have occurred. In view of glacial component

being a smaller fraction and that the ages refer to

events occurring 10–12 ka after the Last Glacial

Maximum (LGM), it is considered that lowering of

groundwater reflects a phase of reduced precipitation/

monsoon regime. Based on the studies on ground-

water calcrete in the central Ganga Plain, Agarwal et

al. (1992) suggested a significant lowering of the

water table during the Mid-Holocene.

There is evidence of tens of kilometre-scale undu-

lations in the study area, producing higher parts with

2–10m relief. These undulations were also responsible

for formation of centripetal drainage basins with ponds

of various sizes (Srivastava, 1998). The field evidence

for kilometre-scale warping and undulations includes

centripetal drainage basins, warped areas undergoing

gully erosion, sudden disappearance of abandoned

channel segments, positioning of fluvial channels with

Bhur sand ridges on high ground and several-metres

high levees adjacent to arcuate ponds (earlier chan-

nels). It has been argued that tectonic activity produc-

ing regional warping took place in pulses between 8

and 5 ka and played an important subordinate role to

climatic change in the formation of lakes and ponds

(Singh, 2001, 2002). Widespread tectonic activity in

Ganga Plain during the Mid-Holocene has been docu-

mented (Singh et al., 1997). The tectonic warping also

affected the ground water table and helped formation of

calcrete bands in warped areas around 5 ka. This also

led to origin of alkaline soils in the upland interfluve

areas (Singh, 2002).

Singh (1996) argued that the channel abandonment

during 8–6 ka could be linked to increasing base level

of the main rivers in response to mid-Holocene high

sea level. The dense network of channels in the central

Ganga Plain formed during lowered base level of the

Late Pleistocene (Singh, 2002).

The tributaries responsible for the formation of

ponds do not show features supporting avulsion. All

the major rivers are incised by 10–20 m in the upland

interfluve surface and possess wide river valleys with

terraces, in which they flow as underfit rivers. These

rivers exhibit evidence of river morphology changes

(river metamorphosis) and shifting within their

respective river valleys. Thus, the smaller river chan-

nels (that eventually transformed to ponds) flowing on

the upland interfluve surface cannot be formed by

avulsion of the major incised rivers.

Based on the foregoing, it is proposed that during

the time span of >13–8 ka, a dense network of

channels developed on the interfluve surface of the

central Ganga Plain. Increased monsoon rainfall in

12–8 ka caused effective sediment–water movement

through this channel network. Tectonic events of

warping during 8–5 ka and increasing aridity caused

disruption of many drainages, especially the smaller

tributaries, forming centripetal drainage areas with

linear ponds and lakes. The larger rivers and some

tributaries were able to cut through the tectonic uplifts

and survived. The weakening of SW monsoon during

middle to late Holocene (Kutzbach, 1981; Van

Campo, 1986; Sirocko et al., 1991; Overpeck et al.,

1996) caused desiccation, segmentation of large linear

Table 3

Chronological studies inferring phases of channel abandonment in

central Ganga Plain

Geomorphic event Present

work

Agarwal

et al.

(1992)a

Singh

(1996)aSrivastava

et al.

(2000)b

Channel

abandonment (ka)

8–6 f 8 f 8 7–5

Channel

activity (ka)

>13–8 – >8 >7

a Based on calibrated 14C-dates on molluscan shells from muddy

sediments overlying immediately above the sandy unit in sequence

of Misa tal.b Based on luminescence dates on alluvial ridges in the central

Ganga Plain.


lakes into a series of ponds. The arid climate and

tectonically enhanced relief caused increased sedi-

ment supply into the ponds around 5 ka. Lowering

of the water table and increased aridity initiated

fragmentation and drying of the ponds.

For the mid-Holocene, evidence from western

Indian lakes indicates lake desiccation f 4.8 ka BP

(Swain et al., 1983; Singh et al., 1990). Increased

dune building activity f 5 ka in the Thar Desert also

indicates a more arid climate in the region (Thomas et

al., 1999). The present inference of a reduced hydro-

logic budget in the mid-Holocene thus accords with

the regional climatic records. The exact timing of the

events in the different regions is however likely to be

different on account of differences in geomorphic

thresholds for lake hydrological changes, dune build-

ing activity, vis-a-vis alluvial processes in the Ganga

valley. Thus, the same overall climatic forcing via the

reduced summer rainfall manifested itself differently

at different spatial locations.

Two dates of 6 ka at 2.2 m and 2 ka at 1.5 m in the

Misa Tal gives additional information. Sedimentation

of the basal 70 cm (2.2–1.5 m) occurred in 4 ka

(f 17.5 cm/ka) whereas 1.5 m of the sedimentation

occurred in only f 2 ka (i.e. 75 cm/ka). Such a

fourfold enhanced siltation rate during the past 2 ka

should be due to enhanced erosion linked to anthro-

pogenic activity. Archaeological evidence in the form

of habitational mounds adjacent to Misa Tal indicates

human settlement at Misa Tal at least for 1700 years

(unpublished data).

In the study region of the Ganga–Gomati inter-

fluve, few archaeological sites are dated around 3.5

ka. However, large-scale inhabitation of this region

was initiated between 3 and 2.5 ka and continued later

without interruption. Most of these habitation sites are

located on the mounds adjacent to the ponds (Singh,

2002). The agricultural activity of these settlers must

have resulted in increased rates of soil erosion and

silting up of the lakes and ponds. At Sarai Nahar Rai,

an archaeological site about 100 km east of the area,

palynological studies of lake sediments revealed

cereal pollen grains in the middle to late Holocene

(f 4.5 ka BP) (Gupta, 1976). The archaeological

studies indicate that during the late Holocene human

activity in this region increased to the extent that such

processes overtook the landscape changes caused by

climate alone.

In summary, the present studies show that in the

central Ganga Plain, formation and evolution of

microgeomorphological features (ponds) was con-

trolled by changes in the hydrological regime, which

in turn was climate/monsoon controlled. Tectonic

activity and base level changes played subsidiary,

supportive roles. In some cases, e.g. at Misa Tal,

human activity became the key factor in the landscape

evolution (siltation), overtaking the climate-induced

changes of the last about 2 ka. The sequence of events

in the evolution of ponds was as follows:

1. Around 13 ka with climate shifting towards a warm

and humid phase, the surface water budget should

have been higher due to glacial melting of

Himalayan glaciers and reestablishment of SW

monsoon during k13–11 ka. This resulted in the

formation of numerous subwatersheds with high

stream frequency and density on the interfluve

surface. Such conditions prevailed till 8 ka.

2. During 8–6 ka, large lakes were formed due to

disruption of drainage system due to tectonic

warping and higher rainfall.

3. During 6–5 ka, increased aridity set in that

enhanced channel abandonment. The water table

lowered and as a consequence a large number of

small subwatersheds and small channels (that

primarily survived on monsoon rains for their

water budget), decayed, forming smaller ponds.

The Monsoon, though weaker, was able to support

these water bodies, at least seasonally. With

increasing aridity, deposition of aeolian sand over

the exposed point bars of abandoned channels

occurred leading to the formation of the alluvial

ridges in the region.

4. Around 5 ka, aridity was accentuated and the

ponds started shrinking and becoming abandoned

due to increased rates of siltation. Enhancement of

relief due to tectonic activity accelerated the

erosion in the watersheds of the ponds. Siltation

of ponds was further accelerated by human impacts

that became significant by f 2 ka.

7. Conclusions

The present study demonstrates that in certain sit-

uations, even microgeomorphological features respond


to global changes and that their sedimentary succes-

sions can help palaeoclimatic reconstruction. The

ponds in the central Ganga Plain developed due to

disruption of drainage systems, in response to climatic

(monsoon) changes, supported by tectonic activity and

possible base level changes. Lowering of the water

table and increased aridity in the Gangetic Plains

caused further fragmentation and desiccation of the

ponds. These events occurred during 8–5 ka and have a

clear monsoon connection. The enhanced siltation of

ponds during the last 2 ka indicates human impact and

increased erosion in the central Ganga Plain.

Acknowledgements

PS and SS acknowledge Council of Scientific and

Industrial Research, New Delhi, for research fellow-

ship. Dr. A.K. Tangri, Uttar Pradesh Remote Sensing

Application Centre, Lucknow is thanked for help with

the remote sensing data. Constructive comments by

Prof. Vishwas Kale, Pune, Prof. Avijit Gupta and an

anonymous reviewer helped in improving the pre-

sentation. UKS thanks the Department of Geology,

Kumaun University for working facilities.

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