Discharge and sediment loads in the Boise River drainage basin, Idaho 1939-40

The Boise River project is a highly developed agricultural area comprising some
520 square miles of valley and bench lands in southwestern Idaho. Water for
irrigation is obtained from the Boise River and its tributaries which are regulated
by storage in Arrow Rock and Deer Flat reservoirs. Distribution of water to the
farms is effected by 27 principal canals and several small farm laterals which
divert directly from the river. The- New York Canal, which is the largest, not
only supplies water to smaller canals and farm laterals, but also is used to fill
Deer Flat Reservoir near Nampa from which water is furnished to farms in the
lower valley. During the past 15 years maintenance costs in a number of those
canals have increased due to deposition of sediment in them and in the river
channel itself below the mouth of Moore Creek.

Interest in determining the runoff and sediment loads from certain areas in the
Boise River drainage basin led to an investigation by the Flood Control Coordinating
Committee of the Department of Agriculture. Measurements of daily
discharge and sediments loads were made by the Geological Survey at 13 stations
in the drainage basin during the 18-month period ended June 30, 1940. The
stations were on streams in areas having different kinds of vegetative cover and
subjected to different kinds of land-use practice. Data obtained during the investigation
furnish a basis for certain comparisons of runoff and sediment loads
from several areas arid for several periods of time.

Runoff measured at stations on the. Boise River near Twin Springs and on
Moore Creek near Arrow Rock was smaller during 1939 than during 1940 and was
below the average annual runoff for the period of available record. Runoff
measured at the other stations on the project also was smaller during 1939 than
during 1940 and probably did not exceed the average for the previous 25 years.

The sediment loads measured during the spring runoff in 1939 were smaller
at most stations than those measured during the spring runoff in 1940. At those
stations where the flow was not affected, or only slightly affected, by upstream
diversions or by placer-mining operations, the largest sadiment loads per unit of
drainage area were measured in Grouse Creek during both 1939 and 1940, amounting
to 3,460 and 2,490 tons per square mile, respectively, and the smallest loads
per unit of drainage area were measured in Bannock Creek during 1939 and in the
Boise River near Twin Springs during 1940, amounting to 14 and 83 tons per
square mile, respectively.

Size anaylses of a large number of samples of suspended and deposited
sediments give an indication of the origin of sediments carried past some of the
stations. The analyses show that most of the sediment measured at the five
stations in the Moore Creek drainages basin above Idaho City consisted largely of
coarse material. They show, also, that the sediment measured at the station on
Moore Creek above Thorn Creek consisted almost entirely of fine material during
practically the entire period of the investigation. Most of the coarse material passing
the stations above Idaho City probably was retained behind the dikes or in
the pools usually formed by tailings from dredging operations in the placer-mining
area below Idaho City, and much of the fine material measured at the station
on Moore Creek above Thorn Creek probably was contributed by placer-mining
activity. During the years when the spring runoff is greater than that measured
during 1939 and 1940, it is probable that the dikes and pools will be less effective
in retaining coarse sediments within the placered area.

Records of sediment loads measured in the New York Canal indicate that a
negligible amount of sediment was deposited there during 1939, but that in 1940
from 10 to 15 percent of the total load at the gaging station consisted of coarse
sediment which was later deposited on the canal bottom. Most of the fine material
was doubtless carried through the canal and eventually deposited in diversion
ditches and on farm land. Because the sediment carried past the station on Moore
Creek above Thorn Creek consisted almost entirely of fine material, it is probable,
that a considerable part of the coarse sediment carried in the New York Canal
during the 1940 spring runoff period was scoured from the large bed of deposited
material in the Boise River above Diversion- Dam, and that the remainder came
from Grimes Creek. Arrow Rock Reservoir was not sluiced during the investigation,
and it is therefore unlikely that any of the coarse sediment in the New York
Canal came from the Boise River above Moore Creek during 1939 and 1940.

The average dry weight of 71 samples of deposited sediments collected from
several parts of the Boise River drainage basin is about 90 pounds per cubic
foot. The average specific gravity of 77 samples of deposited sediments is 2.57.