8.0  Model Description    8.3.2  Reservoir Operations    8.3.3  Groundwater Sources    8.3.4  Instream Flow Requirements    8.3.5  Demand Forecasts    8.3.6  Conservation    8.3.7  TSI Intertie    8.3.8  Future Sources    8.3.9  Hydrologic Data

This section provides an overview of the Tacoma water supply system. The topics of this discussion include a summary of the system, reservoir operations, current instream flow requirements, demand forecasts for this area, and future water supply options. A description of the hydrologic data incorporated into CRYSTAL concludes the Seattle information.

The majority of Tacoma’s municipal and industrial water supply is provided by surface water from the Green River.  Although the U.S. Army Corps of Engineers (Corps) completed construction of Howard A. Hanson (HAH) Reservoir (Figure 8.18) on the Green River in 1961, HAH is operated for flood mitigation and fish flow augmentation.  Despite a capacity of 106,000 acre-feet, a maximum storage of approximately 25,000 acre-feet of water is currently retained in HAH, including conservation pool storage of 1,200 acre-feet.  The Corps currently maintains storage levels in HAH near the conservation pool (i.e., as run-of-the-river) between November and March and stores water to augment fish flows between April and October.

Figure 8.18 Green River System

Approximately 3 miles downstream from HAH, Tacoma Water diverts up to 72 mgd (113 cfs) to meet M&I demands under the city’s first diversion water right (FDWR).  The North Fork wellfield, located 3 miles north of HAH (Figure 8.18), is used as an alternate source of potable water during winter floods when water in HAH can become highly turbid.  Groundwater pumped from the wellfield is considered to be part of Tacoma's FDWR because of strong hydraulic continuity between the aquifer and flows in the Green River.  Therefore, North Fork wellfield generally is used only during winter floods to replace surface water that is not diverted from the Green River because of excessive turbidity.

Tacoma's ability to utilize Green River water is dependent on the capacity of the existing pipeline that transfers water from the headworks to the distribution system.  The city operates Pipeline 1, which has a maximum capacity of 72 mgd, from the Headworks facility to deliver Green River surface water and North Fork groundwater.

All water that is diverted from the Headworks is transferred to Macmillan Reservoir for treatment and storage prior to distribution.

When demand exceeds the allowable Green River first water right, groundwater is pumped from over 20 "In-town Wells" to supplement available surface water supply.  The In-town Wells, which include the South Tacoma aquifer, the Gravity Wells, and other minor wells located within the service area, supply approximately 10-15% of Tacoma’s annual water demands.

In 1998, Tacoma’s customers consumed an average of 69 mgd on a daily basis, including over 20 mgd for industrial use by the Simpson Kraft Mill.  Although annual demand for water has decreased somewhat since the early 1990s, future demands are projected to range from 110-120 mgd by 2020.  These projections assume that current population growth trends continue and that additional purveyors within the county urban growth area are supplied by Tacoma Water.

The operation of HAH and the diversion of water under Tacoma’s first water right have altered the natural flow regime on the Green River.  Flows are monitored on the Green River at the Palmer (USGS-1210-6700) and Auburn (USGS-1211-3000) gauges, located 4 and 32 miles downstream from HAH, respectively. Although flows at these gauges do not currently affect Tacoma's diversions, state-specified instream flows must be met before additional water rights from the Green River are approved.  The fish flow requirements vary seasonally, with greater flow requirements specified during winter and less stringent flows required during summer and fall.  During periods of low flow, Tacoma Water works with the Corps and the Muckleshoot Indian Tribe (MIT) to balance the needs of municipal customers and potentially-affected fish stocks.  Tacoma recently negotiated an agreement with MIT that specifies larger flows during sensitive periods. These flows have been incorporated into the Habitat Conservation Plan.

Table of Contents    8.1.1  Everett Supply System    8.2.1  Seattle Supply System