This report presents Arc/Info GIS maps of historic (pre-European settlement, or approximately
1860) channels and wetlands in the Skagit-Samish delta and Stillaguamish estuary,
and the Snohomish River valley and explains the methods used to create the maps.
Primary sources of information are: (1) U. S. Coast & Geodetic Survey (USC&GS)
charts from 1884 to 1893 (Figure 3); (2) General Land Office (GLO) maps from 1866 to
1877; (3) U. S. Army Corps of Engineers maps; (4) topographic maps; (5) field notes
from the GLO surveys; (6) soil surveys, from as early as 1909; (7) government reports,
and (8) accounts by settlers. The GLO field notes were an especially important source of
information supplemental to map sources. At over 800 points, the notes provided diameter,
species, and distance to nearly 1,400 witness trees; general descriptions of vegetation
and hydrology; and quantitative observations on water depths and flooding.
Estuarine wetlands, mapped by use of various sources and methods, were extensive in the
floodplains of each of the three rivers, accounting for at least one-half of land area in
each area. The Snohomish River valley and Skagit-Samish delta also had extensive
freshwater wetlands (freshwater wetlands include riverine-tidal areas in which tidal
backwater augmented flooding effects). Freshwater wetland was more than four times
the extent of estuarine wetlands in the Snohomish and the two were equal in the Skagit-
Samish delta. Field observations from the GLO notes allow estimates of seasonal
inundation in some of the larger marshes. For example, Marshland, a 1,900 ha wetland on
the floodplain of the Snohomish River, was inundated in February 1871 in at least half
and as much as 90% of its area to a depth of more than two-thirds of a meter. Freshwater
wetlands on the Stillaguamish delta were less extensive than on the Skagit-Samish delta
and the Snohomish valley.
Each estuary had numerous distributary and blind-tidal channels. Channel types mapped
are: mainstem, distributary, blind-tidal, connecting, floodplain slough, and tributary. For
each channel type, segments are broken out in the estuarine-emergent, estuarine-scrubshrub,
tidal-freshwater, and freshwater zones and area summed for each. The Skagit-
Samish delta, because of its diverging-spreading form, is dominated by estuarine channels,
while the confined and low-gradient Snohomish River estuary is dominated by tidalfreshwater
distributary channels. Channels in the confined Stillaguamish valley are also
dominantly estuarine but have a relatively small area because of the relatively steep valley.
Diking, ditching, and filling greatly diminished the extent of freshwater and estuarine
wetlands and blind tidal channels on each of the three river deltas. In the Skagit and Stillaguamish
rivers, nearly all wetlands had been diked, drained, and ditched by early in the
20th century. In the Snohomish valley, change was more gradual but nonetheless nearly
all wetlands had been altered by the middle of the 20th century.