Ecological Flow Assessment Techniques for Headwater Reaches

Headwater streams are ecologically significant areas. In Southern Ontario these ecosystems are under increasing stress due to urbanization, water takings and other human activities. Longitudinal connectivity is an important ecological process that should be considered when setting flow targets for headwaters. Hydraulic models are effective tools for assessing connectivity. The one-dimensional (1D) HEC-RAS simulation software is favored in Ontario because it is familiar to the staff of watershed management agencies. Good performance of 1D hydraulic models under low flow conditions has been achieved for a number of stream reaches in Southern Ontario, provided that survey data adequately represents hydraulic controls such as riffle crests. However, 1D models of headwater reaches have been less satisfactory for the purposes of ecological flow assessment. Three challenges have been identified that may contribute to model error including stream complexity, effects of coarse woody debris and spatially variable discharge due to local regions of subsurface flow. These challenges are discussed in the context of proposed fieldwork and analysis methodologies aimed at developing effective techniques for low flow analysis in headwater streams.