Streams and Floods Topics• The Hydrologic Cycle
• Stream Profiles
• Channel Flow and Sheet Flow Basics
• Drainage Basins
• Drainage Patterns
Typical Drainage Patterns
uniform lithology fractured rocks
volcanic or domed uplifts alternating strong vs. weak rocks
Factors Affecting Stream Erosion & Deposition
• Velocity
• Gradient
• Channel Shape and Roughness
• Discharge
Channel Roughness
Smooth, semi-circular channel yields highest velocity
Wide, shallow channel increases friction
Rough channel also slows river at base
Channel Width and Obstructions
Width change due toStrength change
Channel narrows -> increases stream velocity
Stream Discharge Defined
Discharge (cfs) = average stream width (ft) •
average depth (ft) •average velocity (ft/sec)
Example: Stream dimensions are 100 ft wide and 15 ft deepFlowing at 6 ft/sec (4 mph) has a discharge of 9000 cfs
Humid climates discharge typically increases downstream:1) Water flows into stream from ground2) Tributaries are added downstream
Volume of water that flows past a given point per unit time
Stream Erosion• Hydraulic Action• Stream Transportation of Sediment– Dissolved Load– Suspended Load– Bed Load
• Transportation Mechanisms– Traction– Saltation
Stream Deposition
• Bars• Braided Streams• Meandering Streams and Point Bars
• Flood Plains• Deltas• Alluvial Fans
Deposition and Erosion of River Bars
Flood: Increaseddischarge anderosion!
Normal Flow
Post-flood; newbars deposited
Delta Formation and Morphology• Deltas form when a stream or river flow into a quiet body of water (usually a large lake or ocean).
• Surface marked by distributaries– Small shifting channels that carry water away from main river channel
– Sediment deposited at mouth of eventually block flow causing shift of deposition
• Internal morphology dominated by topset-cross-beds-bottomset sequences
• External morphology depends on environment
Stream Valley Development• Downcutting and Base Level• The Concept of a Graded Stream• Lateral Erosion• Headward Erosion and Stream Piracy
Stream Base Level
Downcutting: Process of deepening a stream valley by erosionof the stream bed. Evolution from narrow slot to wider V-shapedvalley.
Base Level: Defined as the limit of downcutting for a stream.Changes in base level alter the dynamic equilibrium of the system.
Ungraded vs. Graded Streams
Graded stream: Balance between transport capacity and sediment load.Maintained by altering the shape of the channel and stream profile.
Graded Streams• Dynamic equilibrium:
– Balance between sediment load and transport capacity
• Increased stream gradient:– Increases velocity, which allows the stream to carry more sediment and larger particles. This causes more erosion lowering the gradient.
• Change in sediment load:– Decrease load, can cause erosion, again tending to lower the gradient. Common downstream of dams. Response of system is NOT always predictable!
Top Related