Forest stream hydrology is greatly affected by logging practices and FSR construction. From water depths and velocities to changes in hyporheic and groundwater exchanges, stream-dwelling salmonids and their habitat are affected. Modification of this regime goes on to affect stream reaches at different times throughout the year, ultimately affecting characteristics of salmon habitat (Hinch, 2005).
Removal of timber reduces groundwater transpiration and rainfall interception, which often leads to increased runoff within clearcut zones as a result of impermeable surfaces in the watershed due to soil compaction, to road construction, the use of heavy equipment, landing sites, and reduced soil water storage capacity (Hinch, 2005; Chamberlin, 1991). Without a riparian buffer this issue is exacerbated and may lead to mass sedimentation due to increased flow velocity and scouring. Furthermore, increasingly low springtime and summer flows and increased autumn and winter flows exaggerate these effects given the annual growth of peak flows (Bosch, 1982; Harr, 1983). In western North America, these flows are affected by snowmelt, influencing water temperature and flow timing (Pike et al., 2010).
The alterations are generally seasonal and daily streamflows may lead to increased incidences of scour, which impact salmon severely (Rand et al., 2006; Beechie et al., 2006). According to the research raised by Seiler (2003), annual flood magnitude could be strong predictor of both freshwater survival rates and total lifecycle return rates for salmon. The extreme hydrological events (such as floods) during the salmon life stage of egg incubation can be a limitation to egg-to-fry rates by scouring redds and crushing eggs with mobilized gravels or depositing fine sediments on redds, which can be a reduction in available oxygen for salmon (Montgomery et al., 1996; DeVries, 1997). Furthermore, increases in streamflow will lead to a reduction in the distinctness between habitat units (e.g. pools and riffles), which could affect the availability and effectiveness of fish cover habitat, and alter aspects of territoriality and feeding. Apart from that, the availability of slow-water habitats can also be reduced because of the switch and increase of peak flows. Such consequences will potentially lead to a significant reduction in preferred habitats for downstream juveniles, and subsequently a decrease in freshwater survival rates (Latterell et al. 1998). The sensitivity to hydrologic conditions is further supported by a measured inverse correlation between chinook fry populations during the summer in some rivers of the northwest coast and maximum peak discharge of the previous winter. Thus small increases in the frequency or magnitude of peak discharges could have an effect on survival of eggs and alevins. Information is needed on the distribution and significance of forestry induced hydrologic changes and their effects on spawning bed stability.