The impact of urbanization on stream habitat in Lower Mainland British Columbia

Low-order streams in Lower Mainland British Columbia have been surveyed in order to determine their response to urbanization, and to attempt to define limit levels of watershed development. The study watersheds span a gradient of percent total impervious area (%TIA). It was hypothesized that the quality of the following physical elements of fish habitat would be dependent on the %TIA: base flow, cross-sectional geometry, bed particle composition, intragravel dissolved oxygen, riparian integrity, instream cover (large woody debris and rooted cutbanks), bank stability, and temperature. Some of the fish habitat conditions studied were found to improve with increasing urbanization. The urban streams had less fine material and larger coarse material in their beds. Bed coarsening was accompanied with a slight increase in the levels of intragravel dissolved oxygen. The absence of fine material indicates that the study streams, developed approximately 20 years ago, have recovered from the construction and channel adjustment phases of urbanization. The degradation of other elements of fish habitat indicates that the most severe damage is done to streams at low levels of urbanization. Base flow became uniformly low between 20-40% TIA, and caused a decrease in velocity, rather than in wetted depth. The high-gradient study streams widened when the TIA increased to 10-15%. Large woody debris abundance was low in all streams with > 20% TIA. The loss of riparian integrity and large woody debris with increasing urbanization also played a role - stream erosion increased when the buffer strip was less than 30 m wide, and when there were fewer than 5-10 pieces of LWD per 100 m. Since stream degradation takes place at low levels of imperviousness, it is recommended that the Land Development Guidelines increase the required buffer strip width to 30 m, even for low-density development. As well, stormwater detention ponds should be constructed before urbanization is begun, with the aim of keeping cumulative excess shear stress to a minimum.Applied Science, Faculty ofCivil Engineering, Department ofGraduat