Read it through once

The most interesting and curious point is the way in which it grows in dense thickets; the main stem is really horizontal and below the water, but it gives off a number of upright stalks. Now every flood will carry in amongst the stalks quantities of silt and rubbish. Those upright stems will sift the water: all sorts of floating material, sand, silt, dead leaves, fruit, etc., are left amongst them. So that such a marsh or bed of Phragmites is gradually, flood by flood, collecting the deposits of mud, and the bed becomes every year more shallow. At the edge of the marsh there is scarcely any water visible, and grasses and other plants are beginning to grow between the Phragmites stems. Eventually these latter are choked out, and a marshy alluvial flat occupies the site of the old reed-bed.