Dynamics and Vulnerability of Delta Systems - loicz

CS7: Puget Sound Deltas: habitat loss for Chinook salmon
W. Gregory Hood
Throughout the world, loss of native ecosystems through agricultural and industrial development has
impacted fisheries, migratory waterfowl, shorebirds, and other wildlife associated with river deltas.
Many impacts have occurred over the past 2000 years, with accelerated conversions of fish and
wildlife habitat in the last three hundred years. In Puget Sound, agricultural and industrial development
occurred only in the past 150 years, so habitat losses are relatively well documented by historical
surveys (1850-1870), maps (1870-1890), and aerial photographs (since 1931). Large river deltas
historically accounted for 90% of tidal wetland area in Puget Sound, with the greater Skagit Delta
alone accounting for 53% of the total (Collins & Sheikh 2005). Agricultural and industrial development
destroyed 80% of the historical tidal wetlands, with greater losses in river deltas and for certain
habitats. For example, estuarine emergent wetlands declined by 63%, but tidal shrub-shrub and tidal
forested wetlands declined by 98% and 92%, respectively (Collins & Sheikh 2005). Consequently,
the ecology of tidal shrub-shrub and forested wetlands is poorly known, as is the ecological consequence
of their disproportionate loss. We can speculate that because the dominant shrub-shrub species
was a nitrogen-fixing plant it likely contributed to high secondary production through herbivore
and detritivore food chains. Current research also suggests that the tidal shrub-shrub ecosystem may
have high value as rearing and refuge habitat by juvenile Chinook salmon, a threatened species.
In addition to direct ecosystem and fish and wildlife habitat impacts there have also been less
obvious indirect impacts. When tidelands are enclosed and drained, tidal prism is removed from
channels outside the dikes. Decreased tidal flushing causes increased sediment accretion in the
remaining channels (Fig. CS7). Because dikes cause seaward reduction in channels as well as
landward channel loss, there has likely been disproportionately higher loss of tidal channels than
marshes (Hood 2004). In spite of the relatively recent history of ecosystem changes in Puget Sound
river deltas, ecological amnesia afflicts almost all Puget Sound residents, even scientists researching
delta ecology and geomorphology. We have generally forgotten how intact Puget Sound ecosystems
appeared, and we have never known how they functioned. How much more complete is our amnesia
in systems with longer histories of human manipulation? How do we sustainably restore ecosystems
and the associated habitats upon which fisheries and wildlife depend if we cannot remember them?
Fig. CS7: Remnant blind tidal channels in Skagit Delta marshes seaward (left-pointing arrows) and farmland
landward (right-pointing arrows) of dikes constructed between 1870 and 1889. Blind tidal channels disappeared
landward of dikes due to fill placement and rerouting drainage to ditches along property boundaries. Seaward of
dikes they filled with resuspended bay sediments due to lost tidal prism. Only one shallow channel remains
seaward of the dikes. The lower right corner of each photo shows a distributary, disconnected from the river by
dikes in the 1950s. Much lower volumes of tide gate-mediated farm drainage, causing the remnant distributary
to fill and narrow with resuspended bay sediments, replaced river discharge. The 1937 photo is courtesy of the
Puget Sound River History Project. Both photos are at the same scale.
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