Floodplain Reunification-The River Restoration Frontier
Janine Castro
1US Fish and Wildlife Service, Vancouver, WA, USA
Channel reconstruction has become fairly conventional, perhaps even routine, in Pacific Northwest (PNW) stream restoration. Over the past several decades, restoration work has sought to increase available aquatic habitat, and specifically salmon habitat, by lengthening channels, decreasing spacing between pools, changing channel width, or adding large wood. As a stream restoration community, we were “channel-centric”, thinking about streams primarily as linear features bound between two banks on the landscape – we even reported our restoration metrics in linear feet. Much of our design time and budget was dedicated to determining the “correct” channel size – not too big, not too small, but just right. Our goal was to create the ideal stable transport channel, able to pass water and sediment, while neither appreciably aggrading nor degrading. Stream slope, cross-sectional area, and roughness were modified to achieve a perfectly balanced channel, and then grade control structures were added as an extra measure to prevent channel incision. In wide alluvial valleys, floodplain dimensions and characteristics, such as elevation, extent, and roughness, were the product of a stable transport channel design. This often resulted in relatively “high and dry” floodplains that only connected to a main channel during moderate to high flow events because sufficient flow had to be contained within a single channel to ensure sediment continuity – the stable channel gold standard.
While floodplains have long been valued for their ability to dissipate flow energy, store flood water, and provide high flow refuge, they are now also recognized as productive food sources for aquatic organisms. Recent research has concluded that fish grow larger and are more vigorous if they spend part of their life on an inundated floodplain, giving them a survival advantage. PNW stream restoration work has been migrating out of the channel and on to the floodplain because most of our aquatic restoration is funded through salmon recovery dollars, and it is becoming apparent that fish need floodplains. We, the restoration community, are taking more of a spatial view of rivers and are reporting number of acres restored, as well as feet of stream treated. Floodplain-focused restoration projects often raise stream beds, add side channels and alcoves, reconnect seasonally inundated wetlands, and increase large wood on floodplains, as well as in channels, with the goal of greater lateral connectivity for longer periods of time. However, floodplains are primarily depositional landforms that accrete through time; without a regular influx of sediment, many floodplains begin to subside. This presents a conundrum – our channel-focused design approach that results in a sediment transport balance is largely incompatible with restoring a river-wetland corridor in a depositional environment. Our challenge is to develop a new suite of tools that expands both our design palette and our restoration techniques to include a broader range of project objectives, especially floodplain reconnection, because achieving sediment balance is no longer the primary factor driving river-wetland corridor restoration designs.