Streamflow gains and losses in the lower Boise River basin, Idaho, 1996-97

Information on streamflow gains and losses
in the lower Boise River Basin is needed by the
Idaho Department of Water Resources to determine recharge to and discharge from the ground-
water system. A method was developed to select
canal and creek reaches such that a minimum of
two reaches were measured in each of 12 different
areas that share a set of common environmental
characteristics. After a large number of environmental characteristics were evaluated, soil type,
land use, and canal density were selected to define
the 12 areas.
Seepage runs were made on 39 irrigation
canal and creek reaches in the lower Boise River
Basin in June-July and September 1996. During
the June-July seepage runs, irrigation canals
gained and lost water, whereas in September, most
reaches lost. No substantial differences were noted
in the median and spread of flow gains and losses
within the 12 areas; therefore, no direct relation
could be defined between seepage and environmental areas.
Seepage runs were made on three reaches of
the lower Boise River in November 1996 to identify flow gains and losses after the irrigation season. The two upstream reaches had net gains,
whereas the most downstream reach, near the confluence with the Snake River, had a net loss. The
total gain to the river from the three reaches was
90.71 cubic feet per second.
Because of potential flooding in March 1997,
water was diverted from the Boise River into the
New York Canal to reduce flows in the river. This
allowed a seepage run on the canal when there
were no irrigation diversions or return flows. Subsequently, two seepage runs were made in March
when flows near Diversion Dam were about 440
and 860 cubic feet per second. Both gains and
losses were measured along the canal, but losses
were dominant. Total loss from the canal during
the first seepage run was -54 cubic feet per second;
during the second, -143 cubic feet per second. Sixteen wells near the canal were measured weekly
from the last week in February through mid-June.
Generally, water levels decreased from February
to mid-April and then increased through June.
Paired wells near the canal indicated downward
movement of water, probably recharge from canal
losses.
Study results indicate that additional seepage
runs are needed on irrigation canals and creeks,
the Boise River, and the New York Canal. Piezometers installed at different depths are needed to better define vertical ground-water movement and
gradients. Additional work is needed to determine
how seepage in canals and streams relates to environmental characteristics.