GLACIALLY MODIFIED DRAINAGE IN GARIBALDI PROVINCIAL PARK, SOUTHERN COAST MOUNTAINS, BC, CANADA

We document how glaciers have affected drainage in the Snowcap Lakes watershed in Garibaldi Provincial Park, British Columbia, during the past millennium. Currently, Upper and Lower Snowcap Lakes drain into the Pitt River watershed. The drainage basin of Glacier Lake is 172 km² in area; 11% of the catchment is covered by glaciers. Previous research suggests that Snowcap Lakes formerly drained to the east into Glacier Lake, which is part of the Lillooet River watershed. It was hypothesized that Thunderclap Glacier advanced across the western outlet of Lower Snowcap Lake sometime during the Little Ice Age, blocking westward flow into Pitt River and raised the level of both Upper and Lower Snowcap Lakes, forcing them to drain over a sill into Glacier Lake. The timing of these events, however, was poorly constrained. The diversion would increase glacier cover in the Glacier Lake watershed to over 15%, and by inference, sediment delivery to the lake. To better constrain the time of the drainage diversion, we recovered percussion and gravity sediment cores from Glacier Lake. The sediments consist of rhythmically laminated silt-clay couplets, which we interpret to be clastic varves. A conifer fragment recovered from 189 cm depth yielded a radiocarbon age of 1040 ± 50 ¹⁴C yr BP [AD 890-1150], which corresponds to a varve count age of AD 1040 from the same depth. Thick varves were deposited in the lake during several periods in the past 1000 years, but by far the thickest varves date from about AD 1870 to AD 1945. Lichemometry and dendrochronology suggest that Griffin Glacier, which flowed into Upper Snowcap Lake during the Little Ice Age, abandoned its outermost moraine in the early 1700s. The glacier constructed its next-outermost moraine later in the same century or in the early 1800s. Presumably, Upper Snowcap drained to the east into Glacier Lake at these times. Overflow into the Glacier Lake watershed ended when Thunderclap Glacier receded from the lower Pitt River outlet, probably in the early 1900s and certainly before AD 1931, which is the date of the earliest aerial photographs. Rapid glacier retreat between 1920 and 1945 elevated sediment delivery to Glacier Lake and more than compensated for the decline in sediment caused by the cessation of overflow from Snowcap Lake.