Downstream variation in stream power

Abstract

Stream power has a significant influence on many form and process attributes of the fluvial system. Using a similar approach to that suggested by D.M. Lawler in 1992 and 1995, a model is developed to explore how stream power might vary in the downstream direction, based on the assumption that the longitudinal profile of the river has an exponential form. No particular discharge is necessarily specified, but near-bankfull flows are regarded as significant morphologically and one such discharge is the focus here. Total stream power per unit channel length ( Ω= γQs) is predicted to peak at an intermediate location, the position of which depends on the ratio b/ β, where b and β are the downstream rates of change of discharge and slope respectively. If b is a relatively conservative quantity, then the form of the longitudinal profile and in particular its degree of concavity will have the major influence on the position of maximum stream power. Specific stream power ( ω= Ω/ w) is even more sensitive to β and is expected to peak closer to the headwaters, about half way between source and the location of the stream power maximum, although this will depend on how channel width ( w) varies with distance from source. The model is tested along three connected streams of different size in the catchment of the River Trent, taking advantage of a relationship between mean annual flood and link magnitude which enables this near-bankfull discharge to be estimated at any point within the network. The model is verified in so far as stream power does attain a mid-basin maximum, but the observed and predicted locations of the maxima do not always correspond, largely because of the distinctive variation in discharge and especially slope along individual pathways. For example, maximum power lies far downstream along the Trent because of the large increase in mid-basin discharge associated with a series of large, closely spaced tributaries. The importance of slope is borne out by the near-headwater locations for specific power maxima, which tend to be further upstream than predicted along all three rivers as a result of the steepness of upper reaches. The variety of geomorphic settings produces deviations from expectation, which has implications for the movement and storage of material in the fluvial system.