Surface-water and suspended-sediment inflow to and outflow from Nutria
Reservoir No. 3 on the Zuni Indian Reservation, western New Mexico,
were calculated. The period of study was March 3, 1994, to September
30, 1995. Total runoff into Nutria Reservoir No. 3 during the study
period was 6,812 acre-feet.
During the study period, 24,310 tons of suspended sediment was transported
into Nutria Reservoir No. 3, and 259 tons of suspended sediment was
transported out of Nutria Reservoir No. 3. Runoff during the study period,
recorded at an upstream gage with 25 years of record, was 137 percent
higher than that for a 19-month period similar to the study period. This
may indicate that suspended-sediment loads transported into Nutria
Reservoir No. 3 also were larger than average. Based on the difference
between inflow and outflow of suspended sediment, 24,050 tons of suspended
sediment was deposited in Nutria Reservoir No. 3. From March 1994 to May
1994 the spillway elevation of Nutria Reservoir No. 3 was raised to allow
water to be diverted into an adjacent reservoir, Nutria Reservoir No. 4.
This flow and sediment transported into Nutria Reservoir No. 4 were not
recorded. If the elevation of the spillway had not been raised, suspended-
sediment loads recorded downstream from Nutria Reservoir No. 3 would have
been larger and the calculated amount of sediment deposited in Nutria
Reservoir would have been smaller.
Of the total suspended-sediment load entering Nutria Reservoir No. 3
during the study period, 94 percent was transported by an arroyo, Garcia
Draw. Garcia Draw drains only 15 percent of the total drainage area of
Nutria Reservoir No. 3 and contributed less than 5 percent of the total
surface runoff to the reservoir.
The average annual amount of sediment deposited in Nutria Reservoir
No. 3 during the study was 15,355 tons. By using a dry-weight density of
99.4 pounds per cubic foot for the deposited sediment, the annual volume
of sediment deposited in Nutria Reservoir No. 3 is 7.09 acre-feet per
year. This number is smaller than the previously reported sedimentation
rate of 15.0 acre-feet per year obtained from past surveys. The capacity
of Nutria Reservoir No. 3 in 1993 was 191 acre-feet. By using the calculated
average sedimentation rate of 7.09 acre-feet per year, Nutria Reservoir
No. 3 could be completely filled approximately 27 years after the 1993
survey.
