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

39purchase of about a hectare of alluvial land would preserve 200 m of key nursery streamwith a population of >1000 juvenile cutthroat trout, 150 adult cutthroat trout, and about50 adult coho and chum salmon. Alternatively, conservation covenants could bepurchased on these critical prime nursery habitats.4.2.2. Flood Control StructuresThe construction of flood control structures within the Lower Mainland, especially withinthe lower Fraser River watershed, has resulted in considerable impacts on anadromousfish such as cutthroat trout. Flood control structure such as dykes, pump houses,hydraulically operated flood boxes, and manual flow control structures can impact oreliminate fish access to important spawning and rearing habitats. Although many ofthese structures were designed to provide for fish passage, it has become evident that fishpassage through and upstream of these structures is far more impeded than waspreviously believed (Thomson et al. 1999).Pump stations are primarily used to pump water from low lying ditches and watercoursesthat surround agricultural and urban areas (e.g. Fraser, Serpentine, and Nicomekl Riversplus their tributaries). Flood boxes provide a flood proofing role by allowing uplandareas to drain into dyked receiving waters, while restricting rising water levels in thedyked waters from flooding areas outside the dykes (Thomson et al. 1999).The Hammersly pumping station located near the mouth of Mountain Slough is a typicalpump station that is operated in the spring during the period of smolt migration. Costsare sufficiently high that most municipalities have not supported replacement of originalpumping stations with “fish friendly” pumps. High smolt mortalities of 70 % have beendocumented by the Department of Fisheries and Oceans at conventional pumps thatoperate in the spring during the out-migration of cutthroat and coho smolts, whereasscreening (where feasible) can reduce mortality to 5 %. Archimedes screw pumps havebeen installed at a few pump stations in Langley and Surrey, which has eliminated fishmortality at Erickson Creek in the Nicomekl River watershed (Hickey and Whyte 1992).Flood gates can be associated with either box culverts or round CSPs (corrugated steelpipes), with different types and configurations of flap gates including sluice type, selfregulating,and hinged. On Mountain Slough, two flood boxes with sluice type flap gateslocated near the Hammersly pumping station pass gravity flow water during the fall,winter, and spring periods around the pumping station. These flood boxes are manuallyoperated top-mounted (sluice gate) structures. Unlike most flood boxes, these gatesremain closed until they are manually opened. These particular sluice gates wereconstructed at a relatively high elevation, presumably to impound additional water in theslough for irrigation withdrawal purposes. The water velocities in these apparentlyundersized box culverts likely result in fish in-migration barriers during the wet wintermonths when there are significant discharges from the slough. Self regulating flap gatesare similar to sluice gates but have a simple mechanical float-actuated water control valvethat requires no external power source. Flap gates of this type are relatively expensiveand are yet unproven in the Pacific Northwest (Thomson and Associates 1999).