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

37detrimental practices or operations continue, including: (1) removal of native vegetationoutside of dykes in those areas that are ephemerally flooded; (2) invasive dredging ofsalmonid-inhabited agricultural drainages; and (3) fish-killing pump stations (Rosenauand Angelo 2005). In addition, over-subscribed water removals for irrigation can impactminimum summer stream flows in some agricultural cutthroat streams.4.2.1. Agricultural DrainagesAs with past forest practices, agricultural land areas were typically logged and cleared tothe banks of streams. However, in striking contrast to historically logged streams,agricultural drainages often lack a zone of riparian trees because mature trees are notallowed to re-establish. Riparian areas of those streams passing through alluvial fans atthe base of hill-slopes have typically re-vegetated with mature trees because soils areoverly sandy for agriculture, although because of past logging and ongoing maintenance,instream LWD is rare. However, in contrast to fans, channels bordering crop or grazinglands are frequently associated with grasses and sporadic shrubs, and rarely have acanopy of riparian trees.Further, intensive agriculture is supported by extensive use of inorganic and organicfertilizers, with generation of animal waste materials that can degrade water quality if notcarefully managed. For example, relatively high concentrations of ammonium andnitrate, along with low dissolved oxygen concentrations, are evident in the lower portionsof Matsqui and Nathan Sloughs, which both have important cutthroat values. Theprovincial Environmental Farm Plan is a step towards reducing these impacts.Today, many of the watercourses that are associated with fields throughout the FraserValley are maintained in a tree-less condition. The wide-spread view of the agriculturalcommunity is that the only value of these watercourses is as drainage ditches, yet they arealso fish habitats. Due to a lack of riparian tree canopy, such tree-less watercoursesreadily infill with reed canary grasses and excessive aquatic vegetation. This in turnresults in municipal drainage maintenance programs, whereby drainages are routinelydredged of infilled vegetation and in some cases, sediments as well. Yet, these drainagesare frequently inhabited with salmonids, including cutthroat trout and coho salmon.Cutthroat are most prevalent in streams passing through agricultural alluvial fans, as wellas those drainages with significant groundwater inputs, such as the extensive drainagesalong Sumas Mountain in the Fraser Valley (S. Barrett pers. comm. 2004). Inwatercourses with little or no groundwater, such as the drainages associated withMountain Slough, cutthroat trout are restricted to alluvial fans in the drainage systemalthough they do in-migrate in winter and out-migrate in spring through primary ditches.In contrast, juvenile coho are much more wide-spread in agricultural drainages,particularly in winter (Slaney and Northcote 2003).The more invasive dredging of vegetation and sediments is harmful to fish habitat, asdocumented by Rees (1956) at a western Washington drainage where Little Bear Creekwas both dredged and channelized. Dredging and channelization resulted in a 97 %reduction in benthic insects in dredged sections which required almost a year to recover.