If you picked stream banks lined with grasses you’re right!
Grasses provide superior bank stabilization. A single grass plant has hundreds of fine, fibrous roots, growing many feet deep.
However, most people think that trees, with their large and woody root systems, provide the best stream bank stabilization. They don’t for several reasons. Their large roots are not nearly as effective at holding soil.
Trees, when near water, are also shallow-rooted and prone to tip over easily. And they shade the ground beneath their canopy, leaving some parts of the bank without any plant growth.
What about trees shading the water and keeping it cool for fish, like trout? The Driftless is blessed with many natural springs and seeps that supply streams continuously with cold water. So shading is not necessary to maintain a quality fishery for trout and other coldwater fishes.