ECOLOGICAL IMPACTS OF DIKES IN THE SKAGIT DELTA

by W. Gregory Hood, Ph.D.
Skagit System Cooperative

When people consider the environmental impacts caused by dike construction and marsh conversion in the Skagit delta, they usually only consider impacts that have occurred on the converted land behind the dikes. Among the obvious impacts to estuarine fish, including juvenile salmonids, are the loss of small tidal channels that get filled in, conversion of larger tidal channels to drainage or irrigation ditches with an associated degradation in water quality, loss of marsh vegetation that forms the base of the nearshore estuarine foodweb, reduced or complete absence of detrital export to the nearshore ecosystem and foodweb, and reduced or complete loss of access to tidal channel habitat by fish and other aquatic organisms.

However, an analysis of historical photos of the Skagit delta marshes shows that there are also less obvious impacts that occur outside the dikes. For example, in the 1950s the Washington Department of Game, now the Washington Department of Fish and Wildlife, constructed dikes that cut Wiley Slough in half and isolated about 200 acres of associated intertidal marsh from tidal and riverine flooding. When a 1956 air photo of the Wiley Slough area is compared with one taken in 2000, GIS analysis shows that more tidal channel habitat was lost outside of the dikes than inside them (Figure 1). The loss inside the dikes was significant, amounting to 16.3 acres of prime channel habitat for juvenile salmonids and other estuarine fish, but 20.5 acres of channel habitat were lost outside of the dikes. In contrast, tidal channels in the South Fork marshes that are distant from dikes and their influence are much more stable and do not show the same losses in channel area between 1956 and 2000.

Near the outlet of the North Fork of the Skagit River similar losses of tidal channel habitat can be documented for several smaller sloughs impacted by dike construction between 1937 and the present (Figure 2). For one slough (NF1) downstream tidal channel loss was 0.4 acres, almost equal to the upstream channel loss of 0.5 acres. For another slough (NF2) downstream loss was 0.8 acres, or about 20% of the upstream loss of 3.9 acres.

Dikes impact downstream slough geomorphology by reducing tidal flushing of the channel. Significant volumes of tidal water no longer flow through cut-off tidal channels because of the lost channel volume cut off behind the dikes as well as lost tidal volume formerly draining off the now isolated marsh surface with each ebb tide. Reduced tidal flushing allows sediments to be deposited in the downstream portions of the bisected tidal channels, and so they fill in (cf Myrick & Leopold 1963, Hume 1991, Zeff 1999).

These three examples of downstream impacts to tidal habitat by dikes show that we can't take the existing tidal marshes outside of the dikes for granted. They are not untouched or pristine habitat. They have changed since the delta began to be diked in the 1890s due to the influence of the dikes on the movement of water, sediment, and large woody debris-and all of these elements interact to shape tidal channel and marsh geomorphology, and consequently the habitat of a great variety of estuarine dependent animals.

The information presented in this article, as well as additional data and analysis on the downstream impacts of dikes on intertidal channels, is in preparation for publication in the journal, "Estuaries."

Myrick, R. M., & L. B. Leopold. 1963.
Hydraulic geometry of a small tidal estuary. Geological Survey, Professional Paper 422-B.

Zeff, M. L. 1999.
Salt marsh tidal channel morphometry: applications for wetland creation and restoration. Restoration Ecology 7:205-211.

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