Upper course

The upper course of a river is characterised by water with temperatures to 15°C and that are rich in oxygen, a narrow riverbed with flow rate faster than 2 m/s, and a rocky or gravelly bottom. The aquatic vegetation found in these conditions are primarily the mosses and diatomaceous algae. Plankton are almost completely absent, and the river is dominated by the benthic organisms well adapted to the fast water flow. The most obvious adaptations are the flattened flatworms, stoneflies and mayflies, while the caddisfly larvae have hooks that enable them to hold on to the substrate.