This are some of the reasons I am moving North-by-Northwest. [Note: I didn't take these. They come from a major metropolitan newspaper in Seattle]
My last job, as an aquatic biologist, involved stream restoration studies. One of the ways to do this was to add rootwads in the river. This slowed the current and provided a pool for fish to hang out and rest in. In West Virginia, if one placed a rootwad the size of the above pic, you would dam the river.