|
Little Colorado River Watershed Coordinating Council
The Little Colorado River occupies much of the northeast
portion of Arizona. There are two distinct
volcanic fields in the basin: one in the west around Flagstaff and
another in the southeast in the White Mountains area. The Little
Colorado River is the main drainage for the basin. The river flows to
the northwest from the White Mountains area, leaving the basin near
Cameron, Arizona. The Little Colorado River Plateau basin is bordered
on the north by Arizona-Utah state line, on the east by the
Arizona-New Mexico state line, on the south by the Mogollon Rim, and
on the west by U.S. Highway 89. The presence of U.S. Highway 89 does
not have any hydrogeologic significance to the basin boundary, but
happens to coincide with the lithologic and tectonic changes in the
aquifer system.
Elevations in the Little Colorado River Plateau basin vary from, to
4,200 feet above mean sea level where the Little Colorado River flows
out of the basin.
Local Aquifers
Local aquifers are of great importance
for domestic water supplies where the three regional aquifers, the D-, N-, and
C-aquifers, are too deep or have unsuitable water quality. The local aquifers
include alluvial deposits, which occur in washes and stream channels throughout
the basin, sedimentary and volcanic rocks, and various sandstones.
The alluvium along the Little Colorado River and its tributaries is an important
local source of water for domestic supplies. Water enters the alluvium as
discharge from the D, N, and C-aquifers, as streamflow infiltration, or as
direct rainfall. In thicker sections the alluvium is a steady source of water,
but smaller washes can go dry because of overuse or drought conditions. Water
quality varies greatly in the alluvial aquifers.
Basaltic volcanic rocks occur along the southeastern and southwestern edges of
the Little Colorado River Plateau basin. The basaltic rocks form an irregular,
eroded surface and vary in thickness from 0 to 3,000 feet. Groundwater is found
in fractures, cinder beds, and heavily weathered zones. In the southeastern
part of Navajo County, the saturated basaltic rocks, together with underlying
sedimentary rocks, locally are known as the Lakeside-Pinetop aquifer. Water
quality is generally very good and water is used for domestic, stock,
irrigation, and Lakeside/Pinetop public supply purposes.
The Bidahochi Formation forms a local aquifer in the northeastern part of
southern Apache County and near St. Johns. The Bidahochi Formation is composed
of sedimentary and volcanic rocks. The formation generally has three members: a
basal sandstone, a middle volcanic unit, and an upper sandstone member.
Principal groundwater withdrawal is from the upper sandstone unit. Recharge is
derived from rainfall onto the outcrop exposures.
Aquifers of the Little Colorado River Plateau basin contain large quantities of
groundwater in storage; however, they are in a sensitive relationship with the
Little Colorado River and its perennial tributaries. Lowering of hydrostatic
heads by excessive groundwater withdrawals may cause some perennial reaches of
the streams to dry up (Mann, 1976).
The C-aquifer is the source of water for Sterling Spring at the head of Oak
Creek in the Verde River basin. Future development of this aquifer and the
limestone aquifer in the Flagstaff area should be preceded by an area-wide
hydrologic study. Local heavy withdrawals from the C-aquifer may also cause
upward shifting of the salt water interface from the evaporates in the Supai
Formation near Joseph City (Mann and Nemecek, 1983).
The D- and C- regional aquifers are still in hydrostatic equilibrium
(steady-state); however, local groundwater sinks or cones of depression are
developing in areas of heavy pumpage (Arizona Department of Water Resources,
1991) such as the paper mill near Snowflake and three of the power plants:
Springerville Generating Station, Coronado Generating Station (St. Johns), and
Cholla Generating Station (Joseph City/Holbrook). The Navajo Generating
Station, near Page, uses surface water from Lake Powell. Water levels in wells
that tap the confined area of the N- aquifer are declining because of heavy
withdrawals for the Black Mesa coal mine slurry pipeline.
|
