Cartographic scale effect on channel slopes and stream power calculations

Abstract

The channel slope is important in determining the stream power. In some research situations the slope is measured by topographical maps that, depending on the scale, can provide differentiated values. The objective of the research here reported was to delineate the implications of the inter-scales differences of channel slopes on the stream power calculation. Slopes of a small river, with a mixed bedrock–alluvial bed, located in a basaltic plateau in south Brazil, were examined. The measured slopes were compared at 1:50,000 and 1:10,000 scales. In the field, the bankfull geometry was surveyed in ten cross sections, for discharge calculation. The slopes measured by maps were used to determine the discharge, using the Gauckler–Manning equation, and to calculate the total and specific stream power. A tendency was verified for the 1:50,000 scale to overestimate the values in reaches of low slopes (<0.05), coinciding with mixed bedrock–alluvial reaches, while it underestimates in the reaches of larger slopes, i.e. knickzones. As larger is the absolute difference of slope between the analyzed scales, the larger is the stream power differential (%), and as smaller the slopes involved the larger will be that differential. In the analysed case, total and specific stream power differentials vary from 14.6 to 313.4%.