Lillooet-Harrison Drainage Basin: Variable Landscapes Within The Coast Mountains

The Lillooet-Harrison watershed (7870 km(2) in area, of which 459 km(2) is glacierized) contains recurring morphological landscape elements that differ in terms of their level of connectivity with Lillooet River. First, ridge tops with low local relief are connected to Lillooet River primarily through dissolved solids moving along subsurface pathways. Second, small glaciers have little direct connection with Lillooet River because of their numerous proximal lakes, whereas larger valley glaciers are directly linked to Lillooet River. Third, hillslopes subject to infrequent, large mass movements are episodically connected, and fourth, debris flow channels are directly linked with valley-bottom rivers. Fifth, mid-valley and lower valley-bottom sites are sediment storage areas that are episodically coupled to the adjacent river channel. The Mount Meager Volcanic Complex dominates the supply of coarse clastic sediment to Lillooet River and is primarily responsible for the rapid rates of Lillooet delta progradation and aggradation. The 6 % of the watershed that is glacierized provides a high proportion of the fine clastic sediments to Lillooet and Harrison lakes, whereas solute production that has little influence on morphological elements is important in assessing the overall rate of landscape denudation in the Lillooet-Harrison watershed. More frequent extreme rainfall events and rapid glacier retreat together with intensified human occupancy in the valley can be expected to increase the coupling of each of the morphological elements to Lillooet River and expedite the movement of water, sediments, and solutes through the watershed.