Analysis of the effect of changing discharge on channel morphology and instream uses in a Braided River, Ohau River, New Zealand

Constant discharges of 26.5, 56.7, 105, 240, and 507 m 3 s −1 were released down the Ohau River from Ohau A power station, and measurements of water depths and mean velocities made along cross sections in a braided section of the channel. Frequency distributions of water depth and velocity are presented both singly and jointly for each discharge; the methods used provide much more information on the changing character of the river than is provided by conventional hydraulic geometry relations. As discharge increases, existing channels become wider, deeper, and faster and frequently merge to become a single larger channel. However, additional channels are generated with the same characteristics as those existing at lower discharges, and the total number of channels at a cross‐section remains constant. Hence the increase of water surface area tends to be by addition of faster, deeper water to a constant area of shallow, slow water (whose location in the river bed changes). This process has the major implication for instream uses such as salmonid spawning in that the area suitable for these uses may remain constant over a wide range of flows. In some respects, then, the braided river is morphologically more stable than a single‐thread river.