Let’s go over some features and characteristics of channels:
- Thalweg: the line tracing the deepest part of the channel
- Meanders: most streams follow a series of bends or loops that are caused by erosion and deposition of sediments:
Sediments are eroded from the outside (convex) side of the bend and deposited on the inside (concave) side. Why is this? Like the way your car’s differential works on a curve allowing the outside tires to spin faster than the inside tires, water going around a bend is faster on the outside than on the inside. Sediment is therefore eroded from the outside of the bend. The water surface is also higher on the outside of a bend (think about spinning a water bucket on the end of a rope – how does the water surface respond?) This causes helical flow, where water spins in a corkscrew bringing sediment to be deposited on the inside of a bend.
The accumulation of sediment on the inside of a bend is known as a point bar and an eroding outer bend is known as a cut bank.
- Pools and Riffles:
Depth is not constant in streams. Most streams show a regular pattern of pools (deep and flow moving) and riffles (shallow and fast moving). This riffle-pool pattern plays an important role in the life cycles of many fish and aquatic macroinvertebrates.
Pools are located the outside of bends, where erosional forces deepen the channel. Riffles are located between bends. But if riffles have fast moving flow, why do riffles not erode and fill up pools? To answer this, consider that erosion ability (shear stress) is proportional to pressure (I.e., depth) and velocity (I.e., slope). At low flows there is a huge difference in slope between pools and riffles, so fine sediment is eroded from the riffles and settles in the pools. But at high flows the differences in slope became evened out. The greater depth in pools pushes large sediment (gravel, cobbles, rocks) out of the pools where it settles on riffles. Next time you wade in a stream, feel the difference in sediment size between pools and riffles. Having a complex channel with a variety of flow conditions and sediment sizes is important for fish and other aquatic life.