The water surface slope is determined by means of gauges placed at the ends of the
reach, say 1 km upstream of the gauging station and 1 km downstream of the gauging station
(in a straight reach; if ∆h is the difference in water levels in a length L of the reach, then
S = ∆h
L . The slope may also be determined by means of flood marks on either side and their
subsequent levelling.