Riparian zones can be pretty rough places for plant life. Despite readily a available water supply, the unpredictable, disturbance-prone nature of these habitats means that static lifeforms such as plants need to be quite adaptable to survive and persist. Some riparian shrubs and trees have adopted a "live fast, die young" strategy for survival. They must be able to cope with things like floods, ice scour, and erosion.
Putting all their energy into quick growth is useful but it also means that many species, like willows and cottonwoods, have relatively weak wood. As you may know, these trees are quite prone to breaking. The same goes for riparian shrubs like dogwoods. After a flood or massive ice breakup, it is not uncommon to find bits and pieces of these woody plants strewn all over, usually jumbled up in a log jam somewhere downstream. Though this may seem disastrous, but looks can be deceiving.
Whereas they all produce seeds, they can also reproduce vegetatively. This is exactly what you are seeing in this picture. A willow branch, ripped from its parent plant, has settled downstream into the mud. Undifferentiated cells under the bark are now producing roots and stems. In time, this may become a whole new willow tree. Provided the branches and logs contain enough living material, a new plant can take root and grow rather quickly. After a heavy flood event, one of these trees or shrubs can suddenly become multiple clones of the same individual. Research has even shown that this form of vegetative reproduction makes for better survival during flood events than that of seedlings.
Restoration practitioners have taken advantage of this adaptation as well. One of the quickest and easiest forms of riparian restoration is the use of live stakes. Live stakes are simply branches (roughly thumb sized in diameter) cut off from a parent plant and driven into the ground. Success is best achieved when the plant is dormant in either early fall or spring. Once the branches awake from dormancy, they begin growing and entire stream banks can be replanted over the span of a couple hours with only a handful of volunteers.