The Development of Geomorphological Complexity and its Influence on Fish Habitat

Fluvial geomorphology plays a pivotal role in determining stream complexity and hydraulic variation, which in turn influence biodiversity (Bartley and Rutherford, 2005). To date, research focusing on the rate by which stream complexity develops through geomorphological change has been limited. Rapid glacial recession within Glacier Bay National Park and Preserve, Alaska, has created a unique opportunity to fill this research gap by studying the development of stream complexity and associated biotic communities over time.\ud Detailed mapping and characterisation of channel geomorphic units (CGUs) within six streams covering a 200-year chronosequence, and subsequent analysis using Hydrosignature software, has identified changes in hydraulic characteristics from younger to older streams. Water depth, current velocity and Froude number decreased with stream age, whilst older streams contained a greater number of slower flowing CGUs. Older streams also contained greater hydraulic variation and smaller CGU size than younger streams.\ud Analysis of the location, orientation and hydraulic characterisation of coarse woody debris has shown that it plays an important role in producing hydraulic and habitat diversity, promoting channel stability (and the creation of pool habitat) and creating velocity shelters, all of which benefit fish populations.\ud Analysis of geomorphological complexity over a chronosequence has enabled the study to evaluate the development of geomorphic composition and hydraulic complexity over time, and identified the importance of coarse woody debris in initiating these changes. Determination of these changes over a large (kilometres) scale provides new information on the process and timescale of riverscape development