Overview
This lecture focuses on river landforms, especially the differences between braided and meandering streams, and explains key features related to flooding and sediment deposition.
Types of Stream Channels
- Braided stream channels have multiple, shifting channels and form in high-gradient, mountainous areas.
- Braided channels carry large, coarse sediment during floods and drop it when flood waters recede, creating gravel bars.
- Meandering streams have a single, well-defined channel and form in low-gradient, flat areas.
- Meandering streams primarily carry finer sediment and are more common in regions like Texas.
Features of Meandering Streams
- Meandering streams curve back and forth, creating features like cut banks (outer bends) and point bars (inner bends).
- Water velocity increases along cut banks causing erosion, and decreases along point bars causing sediment deposition.
- Ox-bow lakes form when meanders are cut off from the main channel.
- Meandering streams help carve wide valleys known as floodplains over hundreds to thousands of years.
Floodplains and Natural Levees
- A floodplain is the valley around a river that gets inundated during floods, lying between two bluffs.
- Natural levees are raised banks formed by repeated flooding and sediment deposition along the river’s edge.
- Each flood adds more sediment to levees, making banks steeper and providing some flood protection.
Deltas and River Evolution
- Deltas form where rivers meet larger bodies of water, causing a drop in velocity and deposition of sediment.
- River deltas, such as the Mississippi’s bird’s foot delta, constantly change location as the river seeks a shorter path to the sea.
- Abandoned delta lobes can sink (subsidence) as they are starved of new sediment.
Human Impact on Rivers and Deltas
- Cities like New Orleans, built on deltas, face subsidence due to lack of new sediment and rising sea levels.
- Upstream dams trap sediment, worsening subsidence even when river channels are controlled.
Key Terms & Definitions
- Braided channel — Stream with multiple, often changing channels carrying coarse sediment in high-gradient areas.
- Meandering stream — Single, winding river channel found in low-gradient areas, forming curved bends.
- Cut bank — The outer edge of a meander where faster water erodes the riverbank.
- Point bar — Sediment deposit on the inner bend of a river where water velocity slows.
- Floodplain — Flat area around a river that floods when water exceeds the channel.
- Natural levee — Raised bank along a river, built by repeated flooding and sediment deposition.
- Delta — Landform at a river's mouth formed by sediment dropped as the river enters standing water.
- Subsidence — Land sinking due to compaction of sediment, often in deltas lacking new deposits.
Action Items / Next Steps
- Review river landform diagrams, focusing on features of meandering streams, levees, and deltas.
- Prepare for further discussion on dams and their effects on rivers in the next lecture.