8.0  Model Description    8.3.1  Tacoma Supply System    8.3.2  Reservoir Operations    8.3.3  Groundwater Sources    8.3.5  Demand Forecasts    8.3.6  Conservation    8.3.7  TSI Intertie    8.3.8  Future Sources    8.3.9  Hydrologic Data

Current Green River Flows

These are the dominant issues associated with instream flows on the Green River:

• Tacoma's First Diversion Water Right
• Tacoma's Second Diversion Water Right
• Corps baseflow requirements at the Palmer gage (USGS 1210-6700)
• Other state-specified baseflows at the Palmer gage
• Other instream flows at the Auburn gage (USGS 1211-3000)

The locations of these gages relative to HAH and Tacoma's water diversion dam are shown in Figure 8.18. Tacoma's first water right of 113 cfs is the most senior water right because surface water withdrawals from the Green River began in 1913.  If adequate water is available, the Corps releases a portion of the inflows from HAH to meet Tacoma’s first water right and the Corps baseflow requirement of 110 cfs at the Palmer gage.  If the Corps baseflow requirements cannot be met from usable inflows, available stored water is used to augment flows particularly during August and September, when average flows in the Green are lowest.

Although Tacoma's second water right is junior to the Washington State Department of Ecology (Ecology)-specified flow requirements at the Palmer gage, both of Tacoma's water rights are senior to the 1989 Ecology-specified flow requirements at the Auburn gage.  The Ecology-specified flows at Palmer vary seasonally and are partially dependent on the determination of "critical year" conditions. During critical periods, recommended baseflows at Palmer decrease.  As specified in WAC 173-509-030, one baseflow is designated for the Auburn gage.  Although this baseflow varies seasonally, there are no alternate "critical" flows. Ecology-specified flows are higher at Auburn due to inflows between Palmer and Auburn (Figure 8.21).

The recent Tacoma-Muckelshoot Indian Tribe (MIT) Agreement specifies greater baseflows at Palmer (Figure 8.22) than those currently stipulated by Ecology. These flows must be met before Tacoma can divert water under their second diversion right.

At the Auburn gage, flows associated with the proposed HAH expansion project (also known as the Additional Water Storage project [AWSP]) have been developed (Figure 8.23).  The new targets specify higher baseflow during the spring months, depending on seasonal conditions.  Because Snotel data currently are not incorporated into CRYSTAL, the logic assumes that if flows are more than 50% above average flows, wet HCP instream flows will be targets, while dry instream inflows will be targets if flows are 50% below average.  Meeting these requirements is contingent on the expansion of HAH. Since these targets are not legally binding, Tacoma has reserved the right to modify these targets as necessary.

The MIT agreement further constrains Auburn baseflows (Figure 8.24) to reflect seasonal flexibility dependent on storage within HAH (i.e., wet, average, dry, or drought conditions).  Reference zones within HAH trigger different baseflows between mid-July through mid-September.

The MIT agreement is contingent on the additional water supply project that will expand HAH and provide more water to augment low fish flows.  Although the current level of storage in HAH (i.e., before the reservoir is expanded) is not sufficient to meet the MIT targets, the CRYSTAL model can evaluate either the previous Ecology-specified baseflows flows or the MIT-agreement flows at Palmer and Auburn.

Tacoma's Draft Habitat Conservation Plan

The MIT agreement is incorporated in to Tacoma’s proposed Habitat Conservation Plan (HCP). In addition to the MIT-agreement flows and the Auburn AWSP flow targets, freshets (i.e., large releases that enhance smolt out-migration) released during April and May have been incorporated into CRYSTAL.  Because the model is primarily concerned with low flow conditions, the freshet volumes reflect dry conditions as outlined in the HCP.

Another stipulation in the HCP is that Tacoma will not withdraw any additional water under its second water right unless instream flows at Auburn are above 400 cfs and the MIT instream flows at Palmer have been met.  This provision effectively restricts the second water right diversion from the Green River during late summer periods.  Tacoma has retained the right to supplement surface water supplies during this period with groundwater from the North Fork well field and the South Tacoma aquifer.

Table of Contents    8.1.1  Everett Supply System    8.2.1  Seattle Supply System