9596

• UCREFRP Catalog Number: 9596
• Author: Ayers Associates.
• Title: Yampa River Basin Research Final Synthesis Report.
• USFW Year: 1999.
• USFW - Doc Type: Fort Collins, CO.
• Copyright Material: NO

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Comparison of natural, historic and future flows in the Yampa River at
Maybell using the Colorado River Decision Support System (CRDSS)

Background

There have been numerous hydrologic analyses of the Yampa River Basin since the inception of
the Recovery Implementation Program for the Endangered Fishes ofthe Upper Colorado River
Basin (Recovery Program). These analyses have relied on available records and monthly time-
step computer models, such as the Colorado River Decision Support System (CRDSS) developed
and operated by the Colorado Water Conservation Board (CWCB).

The CRDSS uses a baseline of historical hydrologic and atmospheric conditions on which an
overlay of simulated future water supply and demand conditions can be analyzed and compared.
The CRDSS period of record for the Yampa River is water-years 1975-1991 (October 1, 1974-
September 30, 1991). Although 17 years is a relatively brief period, it includes several low-
water years representing different historical conditions; these years are the focus of this analysis.

No daily hydrologic models exist or are contemplated in the near future for the Yampa River
Basin. If a daily model were developed, much of the data for historic diversions, return flows,
etc. would have to be synthesized. Regardless of the level of "resolution" , the output of any
hydrologic model is only one tool to assist decision-makers in reconciling competing demands
for water. The CRDSS relies upon the past to predict what is likely to occur in the future. While
it provides a quantitative estimate of water needs, we cannot rely on it for day-to-day operational
flow management decisions.

The CRDSS can predict discharge at a variety of nodes or discrete points in the basin. For the
purposes of this modeling exercise, two nodes were selected: the USGS gage near Maybell,
Colorado, at rivermile 85.8, and immediately above the Maybell Canal Diversion, at about
rivermile 89.1. The gage at Maybell has been in continuous operation since May 1, 1916, with
the longest continuous record and the best rating section for low-flow conditions of any gage in
the lower Yampa River Basin. By comparison, the gage at Deerlodge Park had only 12 years of
record prior its relocation in 1997. The old gage site was frequently exposed to accumulations
of fine sediment which compromised the reliability of its low.;flow record.

The Maybell gage record is not entirely representative of the overall flow conditions in the lower
Yampa River, because it is only about 3 rivermiles below the diversion structure for the Maybell
Canal. The Maybell Canal has significant water rights that allows diversion of most of the river
during very dry conditions. On September 12, 1994, for example, Maybell Canal diverted 25
cubic feet per second (cfs) while the gage measured less than 8 cfs. Return flows from lands
irrigated by the Maybell Canal gradually increase stream flows downstream from the Maybell
gage. Therefore, during the irrigation season, river reaches upstream from the diversion and
downstream from the town of Maybell generally have greater flows than those indicated at the
gage. However, no gage records are available in this reach to reliably estimate daily flows.
Above the Maybell Canal, flows can be synthesized by combining gage records from the Yampa
River at Craig and the Williams Fork at its confluence with the Yampa near Hamilton, Colorado.
However, these two gages have been in operation only since 1984 and provide a daily record for
less than half of the current 17 -year CRDSS period of record. Another means of synthesizing

Appendix B 1