What Holds a Streambank Together?

What holds a streambank together?

One river, two banks. The left one was cleared to the water; the right one kept its buffer. Drag in the water, toward either bank, to throw a splash at it.

Bare bankcleared — steep raw cut

bank ← drag water

100%bank left

0splashes

0chunks lost

Planted bankbuffer intact — gentle rooted slope

drag water → bank

100%bank left

0splashes

0chunks lost

Raw soil. Where your splashes have loosened the bank but nothing has fallen yet.
Old bank line. Where the bank stood before you started.
Roots. They bind the soil, and they can hold up an overhang the bare bank could never keep.
Muddy water. Every chunk that falls in stays in the river as sediment, and it drifts across to the far bank.
Residents. A dusky salamander and a green frog. The rooted slope is somewhere to live; the raw cut is not.

Moving water pulls at a streambank every day. On bare soil each hit carries grains away until the bank is undercut and a chunk calves off — the land above it goes too, and that soil becomes the mud clouding the water for everything downstream. Roots bind the same soil into something that has to be torn rather than washed away, and a bank that keeps its plants keeps the gentle slope it started with instead of being cut back into a bare vertical face. That is why a planted buffer is the cheapest streambank repair there is.