Do trees and shrubs really stabilize streambanks?

Yes.

Woody roots have been shown to increase the strength of the soil. In many agricultural areas streams have been deeply incised creating deep channels with steep vertical banks. Along such streams, tree and shrub roots provide the strength needed to hold them in place. Nothing can completely stop bank erosion as it is a natural process so in time trees will fall into the stream. However, with proper buffer management trees can be harvested before that happens. Grasses may be planted along stream edges but cool-season grass roots seldom grow deeper than 18 inches while native warm-season grass roots may grow as deep as 3-6 feet depending on soils. Grass roots are small and non-woody and many die each year providing little long-term structural strength for the soil. If stream banks are sloped at a ratio of 3:1 or more native grasses may effectively hold them in place. On steeper banks dense stands of small willow cuttings are very effective at holding banks in place. Sandbar willow is especially effective because this species root sprouts developing a dense web or spreading roots that help stabilize banks. Where banks are more than 6 ft in height above base flow levels even woody roots may not be effective, and bank shaping and soil bioengineering approaches are necessary.

In addition to stabilizing banks, trees and shrubs are also very effective at slowing flood waters and trapping flood debris which keeps it out of the farm fields. The trapped debris decomposes on the site and provides carbon for soil development and microbial processes. Slowing flood flows in tributary streams can reduce the peak flows in larger streams and thus reduce the severity of flooding of downstream urban areas.