Spatial variation in the force required to initiate rock movement in 4 upland streams: Implications for estimating disturbance frequencies

Abstract

Lotic models of disturbance generated by floods and spates suffer from 2 main shortcomings: a lack of knowledge regarding the appropriate spatial scale at which to apply models and a poor understanding of the relationship between discharge sizes and actual disturbance frequencies and intensities. Here, we examine the spatial variability in the forces needed to shift rocks and the utility of hydraulic equations that predict critical shear stresses (τ(c)), which are sometimes used to infer disturbance frequencies in streams. We used spring balances to measure directly the forces needed (F(c)) to move rocks up and out of the stream bed in 4 upland streams (Acheron River, Taggerty/Steavenson riv..