We think of streams and rivers as fixed features of the landscape, but they’re not. Trees continually fall into the water because the banks are always moving laterally (unless the streambed is artificially restrained by riprap or levees). A river is constantly shifting because of the way water flows.
Water that flows in a river moves like a corkscrew, twisting in on itself. Water flowing at the bottom of the river is slower because of the friction between the water and the riverbed, while the water on top flows faster. When a river bends, the faster water on the surface pushes against the outer banks and dumps trees into the waterways, while the slower siltier water at the bottom slips to the inner bank and drops some of its sand or gravel.
Most of the waterways we see are controlled by dams, where the high flows are moderated and the trunks are removed from the river. Here we have a natural stream in the spring, sweeping great trees downstream.
This is near the start of our walk, with tree trunks piled at the point of this little instream island.
Next we have this log caught midstream a wee bit upriver.
Big logs collect smaller logs and branches,
and you can see that the water was much higher a few days ago, when the flow around the near side of this tree caught a bole and a lot of small branches.
By midsummer, this torrent of snowmelt will have faded to a babbling brook. This huge trunk will be immovable… until next spring shifts it downstream again. By nature, a stream is a messy, trunk-littered path.
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