Influences of valley form and land use on large river and floodplain habitats in Puget Sound

Abstract

In this paper, we use a system‐wide census of large river and floodplain habitat features to evaluate influences of valley form and land use on salmon habitats along 2,237 km of river in the Puget Sound region of Washington State, USA. We classified the study area by geomorphic process domains to examine differences in natural potential to form floodplain habitats among valley types, and by dominant land cover to examine land use influences on habitat abundance and complexity. We evaluated differences in aquatic habitat among strata in terms of metrics that quantify the length of main channels, side channels, braid channels, and area of wood jams. Among geomorphic process domains, habitat metrics standardized by main channel length were lowest in canyons where there is limited channel migration and less potential to create side channels or braids, and highest in post‐glacial and mountain valleys where island‐braided channels tend to form. Habitat complexity was lower in glacial valleys (generally meandering channels) than in post‐glacial valleys. Habitat abundance and complexity decreased with increasing degree of human influence, with all metrics being highest in areas classified as forested and lowest in areas classified as developed. Using multiple‐year aerial photography, we assessed the ability of our methods to measure habitat changes through time in the Cedar and Elwha Rivers, both of which have recent habitat restoration activity. We were able to parse out sources of habitat improvement or degradation through time, including natural processes, restoration, or development. Our investigation indicates that aerial photography can be an effective and practical method for regional monitoring of status and trends in numerous habitats.

How does habitat restoration influence resilience of salmon populations to climate change?

“…Moreover, development may preclude floodplain restoration in some areas. Nonetheless, most rivers in the region have substantial river length with wide floodplains (Bond et al, 2019; Stefankiv et al, 2019), and floodplain reconnection is likely to be an important restoration action in many river basins.…”

Juvenile Pacific salmonid habitat use in two Puget Sound lowland rivers

“…Both rivers flow through valleys that historically supported extensive forests and wetlands along with diverse main-channel, off-channel riverine, and floodplain habitats Sheikh 2002, 2005). The lower Snoqualmie River and lower Green River flow through glacial valleys and have a meandering channel form, while the middle Green River flows through a postglacial valley and has an anastomosing or island-braided channel form (Collins and Montgomery 2011;Stefankiv et al 2019). Much of the lower Snoqualmie River valley has been converted to agriculture, while much of the lower Green River valley has been isolated from the river by levees or revetments and converted to agriculture and urban infrastructure Sheikh 2002, 2005).…”

Proactive river corridor definition: Recommendations for a process‐based width optimization approach illustrated in the context of the coastal Pacific Northwest

“…In addition to maintaining conveyance area for floodwater and providing accommodation space for accumulating gravel and cobble deposits, floodplain rejuvenation processes are vital to sustaining the diversity of floodplain habitat environments and functions characteristic of healthy gravel‐bedded rivers in the coastal Pacific Northwest. The dynamic interplay between natural lateral channel migration patterns that rejuvenate the floodplain, accretion of fine sediment on the floodplain, and aging of the floodplain forest create the dynamic patch mosaic of floodplain elevations, ages, and vegetation structure necessary for aquatic and riparian life to thrive and for habitat diversity to be maintained (Collins et al, 2012; Latterell et al, 2006; Naiman et al, 2010) (Figure 3), resulting in higher aquatic habitat complexity and diversity in river reaches bounded by forest compared with agriculture and developed land use at the landscape scale (Stefankiv et al, 2019).…”

“…In most cases the goal was to maximize floodplain use for agriculture and related development, leading to a narrow main channel sized only for flood management. These narrow corridors retain very little of their natural habitat diversity (Stefankiv et al, 2019), as brought into sharp focus by the National Marine Fisheries Service Biological Option on effects of the National Flood Insurance Program (NMFS, 2008), and flood management in such corridors is sensitive to failure due to small changes in flood discharges or sediment supply.…”