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Environmental flows characterization for the Nonconsumptive Needs Assessment

The Nature Conservancy; others

As part of the nonconsumptive needs assessments (NCNA) for the 1177 Basin Roundtables, the NCNA Workgroup and Basin Roundtables have set a goal of identifying flows attributes necessary to maintain priority stream reaches. Stream sizes included in this effort are expected to be, at minimum, major tributaries of Colorado’s mainstem rivers and may include tributaries to these tributaries. The NCNA Workgroup recognizes that some streams may not require flow characterizations and others will require highly specific flow quantification. Between these two types are streams that would benefit from flow guidelines, yet are not a sufficiently high priority to merit resource-intensive, site-specific flow quantification.

The development of environmental flow characterizations that can be applied across broad spatial scales is a central challenge facing instream flow programs. In an effort to provide generalized flow guidance to priority reaches without site specific flow assessment, The CDM Technical Team, with the assistance of outside expertise such as The Nature Conservancy and Colorado State University, will develop a generalized characterization of flow needs for major stream types. Stream types will be based on gross differences in hydrology and function. For example, three major hydrologic regimes present in Colorado are snowmelt, ground water discharge and monsoonal rain. 

These major hydrologic types may be subdivided based on stream elevation, gradient, size or other factor that impacts stream function. For example, there may be characterizations for high mountain headwaters (e.g., the Fraser River), plains groundwater fed streams (e.g., the Republican River) or low elevation monsoonal streams (e.g., the Purgatoire River below Trinidad). We anticipate flow characterizations for around 10 stream types.