Watershed scaling of nutrient loading from eroding streambanks

Background/Question/Methods

Streambank erosion can deliver large quantities of sediment and phosphorus (P) to surface waters, leading to water quality impairments. Identifying and protecting streambanks from erosion could therefore contribute to water quality improvement efforts. However, if resources are limited, where in a watershed should those resources be targeted to maximize reduction of sediment and P inputs? In headwater reaches which are numerous and often assumed to have net erosion, or in higher-order downstream reaches with higher lateral migration rates? A few studies have identified spatial patterns of bank erosion within watersheds, but there remains no framework for systematically evaluating whether these case studies are typical or anomalous. In this study, we use the Horton-Strahler laws of drainage network composition to arrive at a generalized scaling relationship between stream order and channel length. We then define an erosion rate per unit channel length that varies functionally with stream order and with adjustable parameters describing eroding length fraction, average bank height, stream power, and compensating deposition.

Results/Conclusions

The product of channel length and the erosion rate expression for each order yields a function that expresses the expected yield of bank-derived sediment to the watershed outlet by stream order. Parameter values for the Horton-Strahler laws are drawn from the literature and from selected watersheds in Iowa. These values are then used to constrain trends in the distribution of erosion across stream orders that would lead to net streambank sources increasing or decreasing with order. Comparison with erosion measurements from selected watersheds in Iowa suggests that sediment and P loads may often be dominated by either headwater channels or high-order reaches, depending strongly on drainage network morphometry.