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3.2.6 Re lacement Reservoir P As a result of the US Bureau of Reclamation's Colorado Big Thompson Project, Senate Document 80 was adopted by the U.S. Senate in 1937 to protect western Colorado water rights interests. Included in Senate Document 80 are provisions to supply junior irrigation and domestic appropriations of water from Green Mountain Reservoir when the flow of the Colorado River and its tributaries are not Buff cient to satisfy senior calls on the river. When a call is placed on the river by holders of senior water rights, the Division 5 Engineer calls for releases from Green Mountain Reservoir to replace water being used by junior water rights protected under Senate Document 80. These releases replace the loss in flow in the river so that the senior water rights are satisfied. During informal Section 7 consultation with the USFWS a number of issues were raised concerning which water rights in the Colorado River basin are protected under Senate Document 80 and how Green Mountain Reservoir releases are administered for replacement purposes. Thus, it was agreed to use the generic term replacement reservoir to identify the replacement vessel in lieu of identifying a specific reservoir or project. 3.2.7 Municipal Returu Flows Return flows from the District's system occur because a significant portion of the water used in a municipal system is not entirely consumed, but rather returns to the river over time as either wastewater treatment plant (WWTP) dischazges, surface runoff from outdoor uses, and as groundwater flow through. the alluvium. Nearly all of the sanitary return flows from the wastewater collection system return to the Colorado River near the confluence with the Gunnison River at Persigo Wash (bottom of 15-mile reach). However, return flows from septic systems and irrigation return to the Colorado River via groundwater and overland flow entering the Colorado River throughout the 15-mile reach; these returns are governed by the proximity, topographical, and groundwater transmissivity relationship of the specific discharge to the Colorado River. In accordance with the RIPRAP, return flows anywhere in the Colorado River system, even if below the 15-mile reach, can be defined as credits. Return flows from the District's municipal water supply system have been evaluated based upon: ^ water used indoors which is retumed to the stream at municipal wastewater treatment plants and which is returned from private septic systems via groundwater ^ surface and groundwater returns of water used outdoors Indoor use is relatively constant year-round and it is primarily non-consumptive, with most of the water delivered to customers returned through the wastewater system. Returns from outdoor use are significantly less due to consumption through evapotranspiration by lawns and native vegetation. CDM Camp Dresser & McKee o:~soa~-i io~noc~neeexu-e.noc B-6