Why Do Some Rivers Split Into Dozens of Channels? Too Much Gravel
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
A river carrying more coarse sediment than it can move keeps dropping it, building bars that force the flow around them, and the result is a shifting web of shallow channels.
What the pattern looks like
Instead of one deep channel the water spreads across a wide bed of gravel and sand, dividing around bars and rejoining, so an aerial view shows a tangle of threads on a pale flat surface. The individual channels are shallow and change position frequently, sometimes within a single flood. Most of the bed is dry in normal conditions and the whole width carries water only during high flows. The bars themselves migrate downstream, which means a map of the channels is out of date almost as soon as it is made.
The conditions that produce it
Four factors appear together wherever the pattern is found:
- •A steep gradient, so the water is moving fast
- •A large supply of coarse sediment, typically gravel
- •Banks of loose material with little vegetation to hold them
- •Highly variable flow, with big floods between low periods
- •More sediment arriving than the flow can carry away
- •Nothing binding the bed, so bars form and shift freely
Why it is not a meander
The two patterns are opposite responses to different loads and confusing them obscures what a river is telling you. A meandering channel carries fine material suspended in the water, has cohesive banks held by silt and roots, and maintains one deep sinuous channel that migrates slowly sideways. The braided form carries coarse material rolling along the bed, has crumbling banks, and abandons and reoccupies channels constantly. A single river frequently does both along its length, braiding in its steep upper reaches where gravel is abundant and meandering across the lowland where only fine material remains.
How a channel moves
Watching the process at a small scale explains why nothing stays put. Water flowing over a gravel bed rolls the coarsest particles along the bottom during high flow, and where the current slows even slightly the largest stones stop and others pile against them, so a bar starts from almost nothing. Once a bar exists it deflects the flow around both sides, which deepens two channels and starves the middle, and the bar grows. A large flood may cut straight across the whole assembly and start again. The pattern is therefore not a stable structure that migrates but a constantly rebuilt one.
Where they are found and why it matters
The pattern is characteristic of glacial forelands, arid mountain fronts and any setting producing more rubble than the water can remove, so the South Island of New Zealand, the Himalayan foothills, Iceland and Alaska all show textbook examples. The practical consequences are serious. Bridges are difficult because the channel that the pier was built in may not be the channel next year. Land beside such a river is unreliable. Ecologically the shifting bare gravel is a specialised habitat, and several bird species nest on it and nowhere else, which is why damming a river upstream and cutting off its gravel supply removes both the pattern and the birds.
The takeaway
A steep gradient, abundant gravel, loose unvegetated banks and wildly variable flow together produce a wide bed of shifting shallow threads rather than one deep channel. It is the opposite response to a meander, which carries fine material between cohesive banks. Cutting off the gravel supply with a dam removes the pattern and the bare gravel habitat that goes with it.