This report presents the results of an investigation of the ground-water
resources of the populated parts of Clark County. Yields adequate for irrigation can be obtained from wells inmost farmed areas in Clark County, Wash.
The total available supply is sufficient for all foreseeable irrigation developments. In a few local areas aquifers are fine-grained, and yields of individual
wells are low.
An enormous ground-water supply is available from a major alluvial aquifer
underlying the flood plain of the Columbia River in the vicinity of Vancouver,
Camas, and Washougal, where the aquifer is recharged, in part, by infiltration
from the river. Yields of individual wells are large, ranging to as much as
4,000 gpm (gallons per minute).
Clark County lies along the western flank of the Cascade Range. in the
structural lowland (Willamette-Puget trough) between those mountains and
the Coast Ranges to the west. The area covered by the report includes the
urban, the suburban, and most of the agricultural lands in the county. These
lands lie on a Series of nearly fiat plains and benches which rise steplike from
the level of the Columbia River (a few feet above sea level) to about 800 feet
above sea level.
Clark County is-drained by the Columbia River (the trunk stream of the
Pacific Northwest) and its tributaries. The Columbia River forms the southern
and western boundaries of the county.
Although the climate of the county is considered to be humid, the precipitation
ranging from about 37 to more than 110 inches annually in various parts of the
county, the unequal seasonal distribution (about 1.5 inches total for ;July and
August in the agricultural area) makes irrigation highly desirable for most
.crops and essential for some specialized crops.
Consolidated rocks of Eocene to Miocene age, chiefly volcanic lava flows and
pyroclastics but including some sedimentary strata, crop out in the foothills of
the Cascades in the eastern part of the county and underlie the younger, unconsolidated rocks in the lowlands to the west
At most places small to moderate quantities of water can be obtained from
fractures in the older consolidated rocks. However, in the populated parts of
the county, these rocks generally are overlain by considerable thicknesses of more
permeable materials, and few wells have been drilled in them. Springs and dug
wells yield an ample domestic supply at a number of outlying farms in the
foothills. The younger (Pliocene to Recent) unconsolidated materials were deposited
chiefly by streams in the basin formed by downwarping of the older rocks. However, some lake deposits and glacial drift also are included. The oldest unit of
this group, the lower member of the Troutdale formation of Pliocene age, consists
chiefly of clay, silt, and fine sand but includes lenses of coarser sand and, rarely,
gravel. The maximum known thickness of the lower member of the Troutdale
formation is about 660 feet. This unit is not a good aquifer because most of
the strata are fine grained. However, at a few places drilled wells have penetrated lenses of coarser grained materials in these deposits and have obtained
small to moderate amounts of water from them.
The upper member of the Troutdale formation consists almost entirely of
lightly to moderately cemented gravel, of which the most striking feature is the
presence of a considerable percentage of quartzite pebbles. The average thickness of the upper member of the Troutdale may originally have been 300 to 400
feet. The member crops out over considerable areas in the county and, where
conditions of topography and exposure are optimum, has beer very deeply
weathered. It is suggested that the upper member of the Troutdale formation
may prove to be of early Pleistocene age. This member is one of the best aquifers
in the county; here, more drilled wells have been completed in this unit than in
any other--most i