Introduction
During the Spring Quarter of 201 1, the UC Davis Ecogeomorphology class [GEL 1351 studied the Chilko-Chilcotin watershed of British Columbia (see papers
on this website). In late August and early September of 2011, the class visited the watershed to conduct a reconnaissance field study of the ecology and geomorphology of the Chilko and Chilcotin kivers. This report is a summary of the key findings ofthis field study.
The Chilko and Chilcotin Rivers are tributary to the Fraser River, which is the largest watershed ofthe Coast Mountains and Interior l>lateau of British Columbia. The Fraser and its tributaries form an important biological corridor for a variety of aquatic and terrestrial organisms, including the largest sockeye run of any river in North America [Northcote and Larkin, 1989). The hydrology of the Chilko-Chilcotin
River system is dominated by two large glacial lakes: Chilko Lake and Taseko Lake. Below these lakes, the rivers alternate between entrenched, bedrock channels and wandering gravel-bed channels [e.g. Church and Rice, 2009) with mid-channel islands and forested floodplains.
Given the rivers' importance for salmonids, other aquatic fauna, and a variety of terrestrial species, we undertook a reconnaissance study ofnumerous islands an confluences along the Chilko and Chilcotin Rivers with the aim of [1) characterizing the physical and geomophological dynamics ofthis wandering gravel-bed river reach and [2) identifying the sources and location ofriverine and riparian biological productivity. Based on our earlier work [reported on this website) we focused our analysis on the alluvial river segments and confluences.
Within the wandering gravel-bed segments we identified two major types of islands: constructive, mid-channel islands that form by the accumulation of woody debris and sediment along a gravel bar, followed by vegetative colonization, and destructive, marginal islands that have been isolated from a mature floodplain by the development and incision ofcutoff channels. The classification of these islands is based on the processes that dominate theirformation. Biologically, constructive islands are characterized by successionally younger and more variable vegetation communities and higher physical complexity of aquatic habitats, including backwaters, eddies, and riffles. In contrast, destructive islands are characterized by more mature vegetation communities and less complex aquatic habitats with higher-velocity flows. As such, we found differences in fish, invertebrate, avian, and mammalian diversity, abundances, and habitat use on each type of island.
We recognize that distinctions between the two types of islands are often blurred, but differences in their geomorphology and associated biological
productivity argue for our first-order clarification. Most previous work on the Chilko-Chilcotin system has focused on the importance ofthe large headwater lakes as a source ofbiological productivity [especially salmonid productivity). While these are important, our work suggests that the physical complexity created by island formation [and degradation) within the alluvial segments of the Chilko-Chilcotin Rivers plays an underappreciated role in supporting aquatic and riparian diversity and productivity within the watershed.