Effects of glacial retreat on proglacial streams and riparian zones in the Coast and North Cascade Mountains

摘要

Proglacial stream development was studied in coastal British Columbia andWashington, focusing on reaches exposed by post-Little Ice Age (LIA) glacier retreat, to address three principal questions: (i) Does the legacy of LIA glaciation influence the evolution of channel morphology? (ii) How long does it take for riparian forest to establish following glacier retreat? (iii) Can newly exposed proglacial streams provide suitable fish habitat? Channel morphologies were identified by field surveys of 69 reaches in 10 catchments. Riparian forest development and potential fish habitat were characterized in those reaches and an additional 22 catchments using GIS analysis. The landscape template imposed by the Quaternary glaciation appears to override most of the modern effects of the LIA in controlling channel-reach morphology. Binary logistic regression analysis identified elevation and time since deglaciation as primary controls on the presence of riparian forest. At higher elevations, establishment of morphologically functional riparian forest could take several centuries, prolonged by channel instability associated with post-LIA sediment inputs. Of the recently deglaciated streams included in this analysis, the majority (86%) of the total length was of suitable gradient for fish and could be accessed either by downstream populations or from adjacent lakes. Predicted maximum weekly average stream temperature (MWAT) indicated that the post-LIA study streams were thermally suitable for cold-water fish. A future scenario of glacier loss would cause a 14% decline in accessible cold-water thermal habitat in post-LIA streams. Decreased summer flows due to glacier retreat could further limit usable habitat by reducing stream depths and wetted perimeters.