Glacially-induced organisation of channel-reach morphology in mountain streams

We examine the spatial distribution of channel-reach morphologies in formerly glaciated mountain drainage basins of coastal British Columbia, Canada. Using field- and geographic information systems-derived data, we show that the local channel slope and the degree of colluvial-alluvial coupling imposed by the glacial valley morphology dictate the spatial organization of channel types. In particular, the complex, glacially induced channel long profile produces characteristic sequences of channel reaches that depart from the downstream succession (colluvial/boulder-cascade/step-pool/rapids/riffle-pool) distinctive of simple unglaciated mountain streams. Typically, the presence of one hanging valley in the river long profile produces and separates two full successions of channel types: a headmost one characterized by an ephemeral/seasonal hydrologic regime and a downstream one, where water runoff is perennial. We document that channel types are well separated in plots of slope versus shear stress, area versus shear stress, and slope versus relative roughness. In agreement with these outcomes, multivariate discriminant analyses coupled with principal component analysis of 98 study reaches yield a highly successful channel-type classification when slope, shear stress, and relative roughness are considered. Success rates, depending on whether or not boulder-cascade reaches are pooled together with step-pools, are 88% and 75%, respectively. Previous work in unglaciated settings has suggested that mountain channels have distinct bed morphology states that vary primarily with slope; our study reveals that even in formerly glaciated valleys, where slope is largely inherited from glacial times, these distinct bed states exist and vary (mostly) with slope, adding considerable strength to this empirical knowledge