Coastal Cutthroat Trout as Sentinels of Lower Mainland Watershed ...

17petroleum products, detergents, and bleach) to small urban streams. These can be highlydamaging to aquatic life, especially young salmonids, including the egg-to-fry stages.Secondly, urbanization and poor land clearing and/or logging practices can increase peakflows to the extent that more bedload is generated, largely from eroding stream banks. Itsdeposition can cause streams to dewater via subsurface flow during summer droughts.Water withdrawals for community water supplies or for agricultural irrigation can alsodewater nursery streams. Unfortunately, the first habitat units to dewater are riffles, andthereafter pools. Thus, juvenile habitat is impacted first under these conditions becauseeven if they are able to disperse into pools that are still charged with groundwater, theyare placed into direct competition with coho fry or provide a food source for largercutthroat. Both these conditions have been observed at Nelson Creek in Coquitlam,where a headwater reach of the stream dewaters during droughts. Moreover, these effectscan occur regardless of a large intact riparian zone or municipal “greenway”. Bycomparison, a before and after study of the effect of a large freshet on juvenile cutthroatand steelhead abundance in the low gradient and less “developed” Salmon River, wasunable to detect any flood-attributable difference in abundance (DeLeeuw 1982). There,a large floodplain absorbed the flood with little physical damage to the channel.Stream flow is a key determinant of cutthroat rearing habitat. Cutthroat parr to adultstages prefer complex bouldery riffles, and pools with adequate depths, velocities andcover, similar to steelhead parr to pre-smolt stages (R. Ptolemy pers. comm. 2004). Thus,minimum summer flows in south coastal British Columbia are considered a majorconstraint to production as emphasized by DeLeeuw and Stuart (1980) and others (R.Ptolemy memorandum to regional staff in 1981). Aside from Region 2 streams listed byDeLeeuw and Stuart (1980) as over-subscribed in water withdrawals, a useful indicatorof good flows for salmonids is provided by a high percent wetted width, which has beenused in BC as a measure of habitat quality (Ward and Associates 2003).A useful measure of habitat quality was generated from 11 trout streams of a broad rangeof sizes by Tennant (1976), which is referred to as the Fixed Percentage Method (Figure5). Velocities, in particular, decrease rapidly at