PHYSICAL LIMNOLOGY AND SEDIMENTATION IN A GLACIAL LAKE

Abstract

The following conclusions stem from a study of limnology and bottom sediment in Garibaldi Lake, a glacial lake in the southern Coast Mountains of British Columbia. Meltwater streams entering the lake cause overflows or underflows depending on local conditions, notably stream temperature and sediment load, temperature distribution in the lake, and apparently the topography at the stream mouth. Conditions vary with season and tune of day. Mixing of stream and lake waters at shallow (less than 35 feet) and moderate (65 to 100 feet) depths could be demonstrated; penetration of stream waters to greater depths could not be precluded. Sediment loads of the two glacial streams entering the lake are relatively low, and persistent underflows forming significant subaqueous channels probably do not occur. The glacial streams, notwithstanding their low sediment content, provide most of the sediment reaching the lake; a minor amount of debris is apparently ice-rafted from nonglacial areas. The bottom sediment consists of rock flour in which clay minerals are rare or absent. Turbidity currents are considered responsible for the occurrence of graded laminae of fine sand and coarse silt in the deep, flat-bottomed part of the lake, up to 2½ miles from the source of this sediment. Subaqueous slumps probably caused the turbidity currents. Absence of visible varves is attributed to (1) low rate of accumulation of suspended sediment, averaging a small fraction of a millimeter a year, (2) a lag of years, rather than months, between the introduction of much of the suspended sediment into the lake and its deposition on the lake floor, and (3) obliteration of any thin and inconspicuous varves by small-scale slumping on the steeper slopes of the lake basin and by the deposits from nonperiodic turbidity currents on the flat lake floor.