Relationship of stream flow regime in the western Lake Superior basin to watershed type characteristics

To test a conceptual model of non-linear response of hydrologic regimes to watershed characteristics, we selected 48 second- and third-order study sites on the North and South Shores of western Lake Superior, MN (USA) using a random-stratified design based on hydrogeomorphic region, fraction mature forest, and fraction watershed storage (lake+wetland area/watershed area). We calculated several commonly used hydrologic indices from discharge and velocity estimates, including daily flow indices, overall flood indices, low flow variables, and ratios or ranges of flow percentiles reflecting the nature of cumulative frequency distributions. Four principal components (PCs) explained 85.9 and 88.6% of the variation of flow metrics among second- and third-order stream sites, respectively. Axes of variation corresponded to a runoff vs. baseflow axis, flow variability, mean flow, and contrasts between flood duration and frequency. Analysis of velocity metrics for third-order streams yielded four PCs corresponding to mean or maximum velocity, Froude number, and inferred shear velocity, as well as spate frequencies vs. intervals associated with different velocity ranges.