The Currency of Erosion
Stream power is the fundamental measure of a river's ability to do geomorphic work — eroding bedrock, transporting sediment, and reshaping landscapes. Introduced by Ralph Bagnold in 1966, the concept elegantly connects discharge, slope, and gravity into a single quantity that predicts everything from channel pattern to long-term incision rates.
Total vs. Specific Power
Total stream power (Ω = ρgQS) measures energy expenditure per unit channel length, while specific stream power (ω = Ω/w) normalizes by channel width to give power per unit bed area. Specific stream power is the better predictor of erosion because it accounts for how concentrated the energy is. A narrow gorge has far higher specific power than a wide floodplain at the same discharge.
Thresholds and Channel Patterns
Stream power governs channel morphology. Below about 10 W/m², rivers are low-energy and tend to be straight or anastomosing. Between 10-100 W/m², meandering dominates. Above ~100 W/m² with coarse sediment supply, braided patterns emerge. These thresholds, while approximate, provide powerful first-order predictions of river behavior across diverse environments.
Landscape Evolution
The stream power incision model (E = KA^mS^n) has become the workhorse of tectonic geomorphology, linking river erosion to uplift rates and climate. By measuring stream power along river profiles, geomorphologists can infer tectonic activity, identify transient landscapes, and reconstruct erosion histories spanning millions of years.