Downstream variation in stream power

Stream power has a significant influence on many form and process attribute s 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 lo ngitudinal profile of the river has an exponential form. No particular disc harge is necessarily specified, but near-bankfull flows are regarded as sig nificant morphologically and one such discharge is the focus here. Total st ream power per unit channel length (Omega = gamma Qs) is predicted to peak at an intermediate location, the position of which depends on the ratio b/b eta, where b and beta are the downstream rates of change of discharge and s lope respectively. If b is a relatively conservative quantity, then the for m of the longitudinal profile and in particular its degree of concavity wil l have the major influence on the position of maximum stream power. Specifi c stream power(omega = Omega/w) is even more sensitive to beta and is expec ted 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 cha nnel width (w) varies with distance from source. The model is tested along three connected streams of different size in the catchment of the River Tre nt, taking advantage of a relationship between mean annual flood and link m agnitude 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 d oes attain a mid-basin maximum, but the observed and predicted locations of the maxima do not always correspond, largely because of the distinctive va riation in discharge and especially slope along individual pathways. For ex ample, maximum power lies far downstream along the Trent because of the lar ge increase in mid-basin discharge associated with a series of large, close ly spaced tributaries. The importance of slope is borne out by the near-hea dwater locations for specific power maxima, which tend to be further upstre am than predicted along all three rivers as a result of the steepness of up per reaches. The variety of geomorphic settings produces deviations from ex pectation, which has implications for the movement and storage of material in the fluvial system. (C) 1999 Elsevier Science B.V. All rights reserved.