8  Model Description    8.2.1  Supply System    8.2.2  Reservoir Operations    8.2.3  Moraine Seepage    8.2.5  Demand Forecasts    8.2.6  Conservation    8.2.7  Future Sources    8.2.8  Hydrologic Data    8.3.7  TSI/SSP

Seattle Public Utilities currently honors non-binding instream flow requirements on the Cedar River at Renton that were part of the Instream Resources Protection Program (IRPP). The Washington State Department of Ecology (DOE) adopted this flow regime in 1979. Currently, the City uses this regime, shown in Figure 8.13, as an operating target.

The CRYSTAL model employs switching criteria to determine "normal" or "critical" flow years. Critical designation years should not occur in more than 1 in 10 years on average. Actual switching criteria have not been established yet, however the model is set up to evaluate possible switching criteria based on reservoir inflow and storage conditions. Currently pumping of dead storage is only allowed during critical years.

The Cedar River Watershed Habitat Conservation Plan (HCP) has proposed a new set of binding instream flow requirements based on a collaborative study program with Seattle, Tribal, State, and Federal resource managers. This instream flow regime is characterized in the March 17, 1997 Agreement in Principal for the Cedar River HCP signed by five cooperative state and federal agencies, ie, NMFS, USFWS, WDFW, WDOE, the Corps, and SPU.  These requirements include a "high normal" flow regime, a "low normal" flow regime, a supplemental flow regime, and a critical flow regime.  In addition, a Cedar River Instream Flow Oversight Commission will be established, which will include all signatories to the agreement.  The purpose of the Commission will be to provide general oversight, coordination, and where specifically authorized, direction regarding the implementation of the Instream Flow Agreement.

The adoption of the proposed HCP instream flow regime is contingent upon HCP approval, which is expected in late 1999.

The location at which the instream flow regime is measured will be relocated from Renton (USGS gage 12119000) to below the dam at Landsburg (USGS gage 12117600).  Achieving the targets below Landsburg will result in a more natural hydrograph for the river, as well as provide a target that can be more directly controlled by City water facility management.  There will be two additional monitoring locations above and below the hydroelectric facilities at river mile 33.7 and 33.2, respectively.  These locations will monitor the down ramping restrictions, which is not modeled in CRYSTAL. Minimum proposed instream flow requirements are illustrated in Figure 8.14.

In general, the proposed instream flow requirements prescribe a high normal curve in 60% of all non-critical years, and a low normal curve in the other 40%.  Also, supplemental flows are provided during important life cycle stages during 70% of years.

In order to implement the high normal flow regime, the following minimum criteria must be met:

• the 30-day antecedent inflow to Morse Lake on Oct 8th averages > 31 cfs as measured at USGS Stream Gage No. 12115000.
• 15-day inflow into Morse Lake on that date averages > 32 cfs as measured at USGS Stream Gage No. 12115000.
• on October 8, Morse lake elevation is > 1541.5 feet.

Since the model runs on historical flow values, critical year designations for the HCP have been pre-determined for years with low flow, and the above frequency rules were upheld.

Although the South Fork Tolt River did not contribute to Seattle’s water supply until 1964, water rights to the Tolt River drainage basin were obtained in 1936 from the Mountain Water Company.  Minimum instream flow requirements on the South Fork Tolt River originated in a 1956 agreement with Department of Fish and Department of Wildlife and went into effect with the completion of the dam.

Seattle City Light obtained a Federal Energy Regulatory Commission (FERC) permit in 1988 to construct a hydroelectric facility between the dam outlet and the South Fork Tolt Regulating Basin.  As a part of their permit application, an Instream Flow Incremental Methodology (IFIM) study was performed.  The results of this study were used in the 1989 Tolt Agreement between the city, regulatory agencies and the Tulalip Tribes.  This agreement sets flows for the current operation of the reservoir and also for the future

Tolt Treatment Facility will alter reservoir operation by allowing further drawdown of the reservoir. The instream flow requirements, shown in Figure 8.15 and Figure 8.16, for the South Fork Tolt River near Carnation are taken at USGS gage 12-1480, 1.6 miles downstream of the South Fork Tolt Reservoir.

Figure 8.16 Tolt Instream Flow Requirements, with Tolt Filtration Plant

Critical years for both rivers are currently triggered in the CRYSTAL model by conditions on the Cedar River.  In other words, the designation of a critical streamflow year is based on Cedar flows and not Tolt Flows. The model is capable of determining critical year status based on inflows and storages, however.

Table of Contents    8.1.1  Everett Supply System    8.3.1  Tacoma Supply System