USING DRAINAGE AREA POWER-LAW RELATIONSHIPS AS A METHOD TO TEST FOR POINTS OF RIVER CAPTURE

The hydraulic geometry of a stream channel can be defined using power law equations linking various stream parameters at a given channel cross section, forming important determinants in how channel shape evolves downstream. These morphometric relationships make up some of the fundamental principles of geomorphology and are formed upon the basis that channel width, depth, and average velocity increase downstream as power-law functions of drainage area. Deviations from these established relationships would suggest non-steady state equilibrium within a system which could be due to processes like isostatic rebound or differential erosion; however, we predict abrupt jumps in the drainage area vs channel length relationship to possibly indicate river capture in regions where geomorphic indicators such as knickpoints, bedrock terraces, underfit river channels, barbed tributaries, and other drainage anomalies are present.

Like many ‘passive’ continental margin sectors worldwide, southwestern Norway exhibits a sharp, seaward-facing topographic escarpment. This high relief, high elevation morphology is considered atypical and has become a subject of intense study. Drainage networks found in this region display prominent evidence for river capture including barbed tributaries, downstream narrowing, wind gaps, and low relief headwaters frequently containing fluvial sediment. Drainage reorganization and river capture have been previously proposed in the area, yet the exact location of if and where a capture event occurred has not been sufficiently investigated. We combine the geomorphic evidence found on the landscape with coding of power-law geomorphic relationships in order to test specific drainages for river capture. We specifically focus on the relationship between drainage area and channel length, because the addition of a captured river, and associated drainage basin, should result in an abrupt jump in drainage area at the capture point. Each anomalous jump in drainage area is then analyzed for geomorphic evidence in order to determine if that area is in fact a capture point, effectively utilizing the hydraulic geometry of a stream, as expressed by the power law equations, to pinpoint locations of river capture event.