Correlations Between Unit Stream Power, Substrate Resistance, and Incision Processes in a Bedrock Stream: Greenbrier River, West Virginia

We hypothesize that unit stream power will be least where river incision can be accomplished by solution and that unit stream power will vary systematically in adjoining reaches as a function of bed lithology and hillslope processes. We test these hypotheses by examining correlations between substrate and unit stream power in a large, actively incising river in a humid climate. The Greenbrier River, West Virginia is a bedrock stream with a catchment area of 3200 km2 and a gradient of 0.001. Five segments varying in length from 635 to 1453 m were chosen for modeling in HEC-RAS on the basis of substrate resistance, incision processes, and hillslope compositions. Three reaches lie where the river flows along strike and two where the river flows across strike through the synclinal Muddy Creek Mountain. Valley walls of the strike-oriented reaches are composed of carbonates and the beds are characterized as (1) sandstone, straight channel (2) limestone, straight and (3) limestone-shale-sandstone, large bend. Muddy Creek Mountain is capped by thick, boulder-generating sandstones. The reaches surveyed near Muddy Creek Mountain are characterized as (1) limestone bed, carbonate valley walls, entrenched below strath (2) limestone bed, sandstone mega-boulders mantling hillslopes and lying in river bed. Selby rock strength scores of the sandstones and limestones in the five reaches are virtually identical (75-to-80), whereas the shale scored 35. Joint spacing is very similar between the sandstones and limestones. But, corrosion features indicate that solution is the dominant incision process atop limestone whereas steps atop sandstone and shale beds indicate plucking and quarrying. An order of magnitude difference in unit stream power was observed between straight reaches flowing atop limestone and sandstone in the strike-oriented reach. In straight, uniform reaches atop limestone unit stream power is 30 W m-2. In comparison, unit stream power atop sandstones upstream and downstream of the limestone outcrop are 250-to-900 W m-2. Unit stream power is 30 W m-2 atop a friable shale that separates the limestone and sandstones. Unit stream power varies systematically along the limestone, shale, sandstone transition; a km-long pool maximizes unit stream power atop the downstream sandstones and minimizes power atop limestones and shales. Pools in single-substrate reaches are closely spaced and display only three-fold decreases in unit stream power. The two segments within the Muddy Creek syncline display a four-fold difference in unit stream power. Unit stream power averages 200 W m-2 in the segment that is incising a strath below carbonate valley walls and 800 W m-2 in the reach with sandstone-capped valley walls. The additional power in the boulder-choked reach is utilized to diminish sandstone boulders atop the river bed; boulders display abrasive sculpting and meter-scale potholes. Unit stream power in the Greenbrier River varies systematically as a function of (i) the co-dependent variables of substrate and incision processes and (ii) sediment contribution from hillslopes. The solubility of limestone has resulted in greatly reduced unit stream power in limestone reaches. Unit stream power changes rapidly at lithologic contacts; power varies an order of magnitude over distances of 10-channel widths when substrate changes in rapid succession from limestone to shale to sandstone.