These bioswales filter storm water by seeping through the plants and dirt so they don't go directly into the river. This reduces the amount of pollution from streets entering the river.
This is the north entrance to the Eastside Esplanade, that goes around the river. You can kind of see the part of the path that floats on the water, straight ahead. Notice the top where the freeway lane just drops off, that was a freeway extension planned in 1970's which was fought by residents and never installed.