David Gardner

I hypothesize that the morphology and evolution of low order stream channels in lowland permafrost regions are principally shaped by short lived, massive floods that occur when thaw lakes drain. Thaw lakes, which form by thawing and collapse of ice rich frozen ground, are known to drain into other lakes or river channels and tributaries. The imprint of thousands of drained basins on these landscapes underscores the possible importance of these events to stream channels. However, the influence of floods on channel and valley morphology has been poorly examined to date. To investigate these effects I use dual frequency differential GPS measurements (accurate to within +/ 5 cm) of valley and drained basin morphology from lower order drainage basins within the Running

River watershed, Yukon Coastal Plain, Canada. The region is characterized by continuous permafrost (MAAT = 10OC) and extensive thaw lake development. Floods from a recently drained thaw lake with water volume of approximately 6.4 x 106 m3 drained in two events (4.8 x 106 m3 and 1.7 x 106 m3 respectively) lasting approximately 0.5 to 2 days each, incised an approximately 15 m deep and by up to 18 m wide and 500 m long channel. In comparison, maximum limit to flood volume from spring melt is 4 x 105 m3, assuming instantaneous melt of snow pack in the entire drainage basin an order of magnitude in difference the measured drained thaw lake. GPS measurements show large flood terraces and deep incision of the current channel on Grayling Creek, a tributary to the Running River with baseflow of approximately 4 m3/s. Sedimentary sections in flood terraces reveal numerous Holocene flood deposits. Massively over fit river valleys are unlikely to have have been incised by the small amount of precipitation and melt water run off that occur in the dry climate of the western Arctic. Thaw lake drainage events produce 20 to 100 times greater discharge (110.7 m3/s) and 2 to 5 times greater stream power per unit width of channel (101.8 J/s). Stream power during thaw lake drainage events cause channel initiation to take place, where the active layer composed of permeable peat is over saturated allowing open flow to take place. Once open flow has been established, stream power causes large rates of rapid incision through the active layer and into the permafrost. Thaw lake drainage has a large affect on river channel morphology and acts as a primary architect in shaping low to intermediate order Arctic rivers and landscapes.

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Pages: 98
Supervisors: Lawrence Plug