9581

• UCREFRP Catalog Number: 9581
• Author: U.S. Bureau of Reclamation.
• Title: Final Environmental Impact Statement - Navajo Reservoir Operations Volume II Technical Appendices.
• USFW Year: 2006.
• USFW - Doc Type: Grand Junction - Durango, CO.
• Copyright Material: NO

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(Mexican Hat). The method consisted of subtracting the monthly natural flows of the
La Plata River, the Mancos River, McElmo Creek, and the CRSS San Juan River near Bluff
from the CRSS natural flow at Archuleta. The net gains and losses in this reach were then
distributed among the intermediate stations along the mainstem of the San Juan River. The
distribution for each reach was calculated as the mean annual gain or loss using the 1970 to
1993 natural flows for the appropriate station set. The distributions, expressed as a
percentage of the total gain or loss by reach, were 0.0% from Archuleta to Farmington,
7.0% from Farmington to Shiprock, 58.7% from Ship rock to Four Comers, and 34.3% from
Four Comers to Mexican Hat. Using these percentages, the monthly gain or loss was
computed for each intermediate station for years 1929 to 1969. For 1970 to 1993 the gain or
loss was found by the difference of Reclamation natUral flows.

The RiverWare model of the San Juan River Basin operates on a monthly time-step,
simulating the flow at every gaging station for various depletion scenarios (current,
depletion base, and various potential future projects). The model determines daily flows for
the simulated Navajo Dam releases and the proposed ALP Project Ridges Basin pumping
plant only. Monthly flows provided insufficient information to adequately describe the
runoff hydrograph (magnitude, duration, timing, and shape) necessary in the flow
recommendation process. Thus, it was necessary to temporally disaggregate monthly flows
to daily flows for the San Juan River mainstem below Navajo Dam. This was achieved by a
daily mass balance on the mains tern computed in a spreadsheet after each RiverWare run.
The daily distribution of natural stream reach gains and losses was estimated using the
difference between daily gage records. Likewise, the gaged flow records for the Animas,
La Plata, and Mancos rivers attheir mouths were used to disaggregate the RiverWare
simulated monthly flow of each river to daily flow. Simulated monthly diversions and
return flows along the mainstem were disaggregated to daily values by distributing the
monthly flows into quarter month values. The distributed quarter month flows were then
uniformly converted to daily flows.

Irrigation diversions, depletions, return flows, trans-basin diversions, and M&I uses were
explicitly represented and modeled in RiverWare for all major San Juan tributaries (San Juan
River above Navajo Dam, Piedra, Los Pinos, Animas, La Plata, and Mancos rivers and
McElmo Creek). All other tributaries were aggregated into the gains and losses to the reach
of the San Juan River into which they flow. The unnatural depletions from these minor
tributaries were treated as direct diversions from the San Juan River. Navajo, Ridges Basin,
Vallecito, and Florida reservoirs and Jackson Gulch were explicit nodes within the model
and their operations were simulated according to rules. Operations of Electra Lake and all
other water impoundments, including stock ponds, were ignored. However, the
evaporation losses from these facilities were included as depletions from their associated
streams.

Several refinements were developed to compensate for peculiarities in the way the natural
flow study handled some depletions and the resulting RiverWare configuration. In the
natural flow study offstream depletions, remote from the mainstem and major tributaries,
were treated as direct diversions from the mainstem. As a result these offstream depletions,
both irrigation and non-irrigation, could call on Navajo Reservoir in the model and
overdraw the reservoir during simulations. By limiting these offstream depletions to the

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