science fora changing world Ground-Water Flooding in Glacial Terrain of Southern Puget Sound, Washington glacial lakes, and diverting drainage landforms and, in some places, eroded southward to the Chehalis River and then away sediments deposited during the west to the Pacific Ocean to create exten­ glacial advance. Coarse sediment, known sive outwash plains6' 7' 10. At its maximum as the Steiiacoom Gravel, was also extent, the glacier stretched from the deposited on the upland by water flowing Cascade Range to the Olympic Mountains through the intersecting channels and and extended south as far as Tenino, braided streams that further conveyed the Wash., in Thurston County, occupying all water away from the proglacial lake.2- 13 of the lowland area and lower mountain This gravel deposit is consistently coarse valleys. The glacier reached altitudes up over the central Pierce County upland to 4,000 feet along the mountain front10; area. Stones in the Steiiacoom Gravel are 6,000 feet near the present day United predominantly 1 inch in size and most do States-Canada border; 3,000 feet near not exceed 3 inches. 13 The thickness of Seattle; 2,200 feet near Tacoma; and less the gravel is generally 20 feet or less with than 1.000 feet near Olympia. 1' 4- 10 a maximum that rarely exceeds 60 feet. The resulting landscape is characterized T^\ ue to a global warming trend, the by many shallow, elongated depressions Figure 1. Proglacial Lake Puyattup and J ^Vashon Glacier began retreating and ice-contact depressions (kettles). The successive Lake Spillways (modified from its terminus about 17.000 years ago.7 larger and deeper depressions are occu­ from Thorson, 1979). As it retreated, meltwater from the glacier pied by ground-water-fed lakes, such as again formed large proglacial lakes. One American and Gravelly Lakes. 13 round-water flooding happens in of the early proglacial lakes that formed G low-lying areas when the water in the Pierce County area was Lake ver time, surface-water streams table rises above the land surface. In the Puyallup9- 13 (fig. 1). Discharge from the O developed over this upland in complex glacial geologic framework of proglacial lake cut outwash channels into many of the braided stream channels at southern Puget Sound, this phenomenon the upland west of the proglacial lake, and the base of the outwash channels (Ohop can occur along former outwash channel its water and coarse sediment were spread Valley Spillway and Ohop Creek; depressions after two or more consecutive over the central and western parts of this Muck Creek Channel and Muck Creek; years of above-average precipitation. It upland region.2- 13 Large volumes of water Clover Creek Channel and Clover Creek; has also occurred and been documented in from the lake flowed the glacial environment found in Illinois. 12 south, originally An examination of the geomorphic history through the Ohop and processes associated with the last Valley Spillway. period of glaciation provides insight on As the ice retreated the causes of this unique and persistent northward lake flooding problem of the southern Puget levels dropped, Sound. and successively lower channel he present topographic shape and spillways were T form of the southern Puget Sound used: Muck Creek Lowland has been largely influenced by and Kirby, Clover the most recent glacial advance and Creek, and South retreat. The passage of the glacier from Tacoma Channels, north to south left distinctive landforms, respectively9' '3 such as ice contact, north-south-trending (fig- 1). depressions, outwash channels, kettles, and drumlins. The last extensive glacia­ he large tion of the southern Puget Sound was the T volumes of Vashon Stade of the Fraser Glaciation. water flowing The glacier advanced from the north into through these the Puget Sound Lowland about 18,000 spillways modi­ A. Ground-water flooding near Clover Creek, April 4, years ago, impounding rivers, creating fied the upland 1997 (photo by A.K. Williamson, USGS1997). reports in residential and industrial lakes are present in the northernmost areas.3- 5-" Little if any information is channel; the only named one is Stony known about the occurrence of flooding Lake. During the flooding event of March in the unpopulated areas. 1997, this lake and the other lakes ex­ panded and flooded homes (insert B) and round-water elevations for the roadways (insert C). The small lake near Garea west of the town of Freder- 192nd Street East expanded during March ickson for March 1997s indicate that the to an area covering about 25 acres, ground-water flow follows the abandoned flooding 192nd Street East (insert C), stream channel and flows southeast to closing the road and remained flooded for northwest toward Spanaway Lake and several months. Water over the roadway local surface-water drainages at an was reported to be about 4 feet deep.5 estimated velocity of 1.000 feet per year.5 Also, normally dry topographic lows This results in a unique phenomenon in within all three abandoned stream some channels often called transient channels flooded (see map and inserts ground-water flooding, in which the A and D). ground-water flooding moves progres­ sively down the outwash channel. nformation on the depth and extent of flooding in the westernmost aban­ he observed ground-water flooding I doned stream channel is minimal because is generally located within aban­ T the area is unpopulated and lies within the B. Ground-water flooding near Stony Lake, April 4, 1997 (photo by A,K. Williamson, doned stream channels west of the Kirby USGS 1997). Fort Lewis Military Boundary. What is Channel. Several small ground-water-fed known is that the 36-inch culverts near and the South Tacorna Channel and Flell flooding, it can take several months for D. Ground-water flooding near Eustis Road, April 4, 1997 (photo by A.K. Creek), except for the area west of the the ground water to recede from the area. Williamson, USGS 1997). Kirby Channel. This outwash channel Many areas of nearby Thurston County never developed surface-water streams; exhibited depression flooding during the instead, it contains many small, shallow, winter of 1996-97. In most cases, the 235th Street East and Highway 7 and near 208th Street East and 8th Avenue East were filled with ground-water-fed lakes and marshes in water levels of these ponded areas were moving water during the flooding in February and the abandoned braided stream channels. above the surrounding water table. March 1999.8 Water also backed up against 208th Under certain conditions, however, when Street East and flooded Highway 7 with 1.5 feet to the ground-water levels rise to elevations ost of the flooding that occurred that intersect and inundate low-lying M in Pierce County during the 2 feet of water, just south of the Highways 7 and 507 junction, closing the road for a short time.8 areas west of the Kirby Channel, ground- winter and spring of 1996-97 is thought to water flooding occurs (fig. 2). This be from ground-water flooding. Anec­ dotal accounts of the general location and he conditions that have been associated phenomenon is hard to distinguish from, T with the incidence of ground-water flooding and is often mistaken for depression extent of the flooding are shown on the map. A complete account of the flooding are sequential (2 or 3) years of above-average ground-water flooding, in which surface precipitation,3- 5-" and cool, wet springs with mild runoff collects in low-lying, low- is unknown. The information available is based on oral communication and local summers. The excess precipitation increases the permeability areas. In both types of recharge to the ground-water system, resulting in higher ground-water levels. Cooler temperatures reduce the amount of evapotranspiration from plants and water bodies, which helps to maintain the higher ground-water levels. These factors, combined with the hydrogeologic properties of the glacial deposits, lead to increased ground-water storage. When the total available ground-water storage is exceeded, ground-water flooding o:curs. ther conditions that may also contribu.2 O to ground-water flooding are based on changes in land use. The conversion of undevel­ oped forested areas into developed commercial and residential areas potentially reduces the evapotrans­ piration and may increase the rate and amount of precipitation available to recharge the ground-water system through septic systems, irrigation, and certain types of drainage systems. C. Ground-water flooding over 192nd Street East, April 4, 1997 (photo byA.K. Figure 2. Known areas and approximate extent of ground-water flooding, Pierce County, Washington, Williamson, USGS 1997). 1996. Arrows show the general direction of regional ground-water movement. SELECTED REFERENCES 'Blunt, D.J., Easterbrook, D.J., and 5Hart Crowser, 1997, Flooding assessment, 10___1980, Ice-sheet glaciation of the Rutter, N.W., 1987, Chronology of 192nd Street East and Canyon Road, Puget Lowland, Washington, during the Pleistocene sediments in the Puget Frederickson, Washington: Seattle, Wash., Vashon Stade (late Pleistocene): Quater­ Lowland, Washington, in Schuster, I.E., Hart Crowser. Report J-4736, 16 p. nary Research, v. 13, p. 303-321. ed., Selected papers on the geology of Washington: Olympia, Wash., 6Noble, J.B., and Wallace, E.F., 1966, "U.S. Army Corps of Engineers, 1997, Washington Division of Geology and Geology and ground-water resources of Post event report Winter storm of 1996- Earth Resources Bulletin 77, p. 321-353. Thurston County, Washington: Olympia, 97, Federal Disaster DR1159, Western Wash., Washington Division of Water Washington Summary: Seattle. Wash., 2Bretz, JH.. 1913, Glaciation of the Resources Water Supply Bulletin 10, Federal Emergency Management Agency, Puget Sound region: Washington v.