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The Lost River
and Birch Creek playas occupy a portion of ancient Lake Terreton,
which, under the cooler, wetter conditions of the late
Pleistocene, covered approximately 90 km2 of the
northern half of the INEEL. Mud
Lake is a mere remnant of Lake Terreton, the shoreline of which
was roughly coincident with the 4,800-foot (1,463 m) contour
(Hackett and Smith 1992). During
most of the Holocene, the playas of the Lost Rivers and Birch
Creek formed extensive wetland areas that likely would have
supported a diversity of plants and animals.
Now, as a result of extensive upstream irrigation
diversions, which began in the 1880’s, water flows into the
sinks only during years when precipitation is well above normal.
During the wet period of the early 1980’s, much of the Big Lost
River/Birch Creek wetlands were flooded.
Drought followed, and the summer of 1993 was the first time
in 7 years that flow from the Big Lost River actually reached the
sinks, and then only for a few days.
However, during June and July of 1995 water flowed into the
sinks for several weeks. As
noted earlier, June of 1995 was the wettest month of record at the
INEEL.
The need for flood control was
recognized during the 1950’s because of the potential for flood water reaching
ICPP and TRA, which had built on the Big Lost River flood plain (see Figure 1).
Flooding resulted from ice jams, which caused the river to overflow.
Frozen soils prevented infiltration, resulting in overland flow.
To address this problem, a small diversion dam and channel were built on
the Big Lost River at its southernmost point (see Figure 1) in 1958.
During winter, water is diverted from the Big Lost River through the
diversion channel into topographic depressions known as Spreading Areas A, B. C.
and D. This prevents water from
flowing north across the INEEL at times when ice jams and frozen soil could cause
flooding at facilities on the flood plain.
During the winter of 1983-84,
wet conditions coupled with extreme cold (-47oF) resulted in ice jams
between Spreading Areas A and B. Water levels rose to within 0.15 m of overtopping the
diversion dam, which threatened flooding at the Radioactive Waste Management
Complex (RWMC). As a result, the
diversion dam and containment dikes were raised several feet and the diversion
channel was enlarged to provide additional flood protection.
Spreading
Areas
Over much of the
INEEL,
surface runoff drains into small playas or low-lying areas between lava ridges.
Some of these local drainage systems are sizable, but they typically flow
only during the spring runoff following wet winters.
No perennial tributary joins the Big Lost River channel. During the
Pleistocene, high discharge seasonal flows from the Lemhi and Big Lost River
mountains formed a series of large alluvial fans that slope eastward from the
foothills along the western side of the INEEL.
These are variously dissected and patterned by meandering depositional
channels. Smaller alluvial fans
occur at the base of Big Southern and Twin Buttes.
Given that surface water at
the INEEL flows into closed basins, it follows that there are only two pathways
for water to leave the INEEL: evapotranspiration or deep drainage into the vast Snake River
aquifer. The water table at the INEEL
typically is between 50 and 270 m below the surface (Link and Phoenix
1994). Where soils are sufficiently
deep to store all of the water received as rain or snow, that water will be
returned to the atmosphere via evapotranspiration during the next growing season
(see Anderson et al. 1987). In most
years, 1 m of soil would suffice to store the water received, and 1.5 to 2 m
would be adequate, even in the wettest years (Anderson et al. 1993).
Thus, recharge of ground water that may eventually reach the aquifer is
limited to playas or depressions where water accumulates and to areas having
shallow soils or basalt outcrops.
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