They form a gradient from fast-moving, shallow water with relatively large particles on the stream bottom (boulders and cobbles) in riffles, through slower deeper areas with smaller particles on the bottom (gravel and sand) in runs, to deep slow water with silt and mud bottoms in pools.
(They; form; a gradient from fast-moving, shallow water with relatively large particles on the stream bottom (boulders and cobbles) in riffles, through slower deeper areas with smaller particles on the bottom (gravel and sand) in runs, to deep slow water with silt and mud bottoms in pools)
They
form
a gradient from fast-moving, shallow water with relatively large particles on the stream bottom (boulders and cobbles) in riffles, through slower deeper areas with smaller particles on the bottom (gravel and sand) in runs, to deep slow water with silt and mud bottoms in pools