CITY OF DES MOINES SHORELINE MASTER PROGRAM

Des Moines Shoreline Inventory and Characterization
partially to completely eroded in places by subsequent glaciations or erosion during interglacial
periods.
The most recent incursion of glacial ice into the central portion of the Lowland is called the
Vashon Stade of the Fraser glaciation, which receded from the area about 13,500 years ago.
Since then, present-day geologic processes, such as erosion and deposition by streams and
landsliding, have modified the ground surface and further complicated the geology. In addition,
fill has been placed across much of the area for constructions of roads, businesses and Sea-Tac
airport.
Most soils exposed at the ground surface within the study area were deposited by the last glacial
episode (Waldron, 1961 and 1962). Lodgment till mantles much of the upland area of Des
Moines (Figure 4) but is generally absent along the steeper portions of the bluff at the edges of
the upland. Lodgment till is an unsorted mixture of sand, gravel, silt, and clay deposited at the
base of a glacier and has been compacted to a very dense state by the great weight of the
overriding ice. Lodgment till has very low permeability and typically acts as an aquitard,
restricting the downward flow of groundwater and reducing recharge of deeper aquifers.
Recessional outwash and recessional lacustrine (lake) deposits of variable thickness commonly
overlie the till. These sediments were deposited in topographic lows in the till surface where
meltwater streams drained from the receding glacier, such as along the headwater areas of Des
Moines and McSorely Creeks (Figure 4). Recent peat and muck deposits have accumulated on
top of these poorly drained recessional lacustrine deposits or on top of recessional outwash
deposits, where downward percolation of groundwater is impeded by the presence of till at
shallow depths beneath the outwash. These areas of peat and muck deposits are also sites of the
larger wetlands within the watershed (Figure 3).
Underlying the till are thick deposits of sand and gravel separated by finer grained layers of clay
and silt or tight, well-graded soils, such as till. These layers comprise several aquifers and
aquitards within the subsurface and control subsurface water movement to the shoreline.
Surface and Groundwater
The Des Moines watershed lies within the South King County Groundwater Management Area
(GWMA). Groundwater and hydrology of the watershed is well described in Luzier (1969) and
Woodward et al. (1995). Additional analysis and groundwater protection planning are being
conducted under King County’s Groundwater Management Program.
The upland surface has several small lakes and numerous streams that flow short distances from
the upland area to the shoreline (Figure 5). Precipitation falling within the watershed is
conveyed directly to lakes and streams by surface runoff or travels in the subsurface as
groundwater flow. Small amounts of rainfall soak into the ground, but during heavy rainfall, the
ground quickly becomes saturated, inhibiting further infiltration. Water that is unable to
infiltrate travels down slope across the ground surface as stormwater runoff. Surface runoff may
erode soil, which is conveyed to streams and eventually to the shoreline of Puget Sound.
Poorly drained areas of the upland plateau are the sites of former or existing wetlands. Wetlands
regulate the flow of water within a watershed by storing water during precipitation events,
slowing the conveyance of water from the upland to the shoreline, and increasing infiltration.
Page 10 March 2005