Tolt River
We are studying water temperature in the Tolt River.
The Issue:
The Tolt River and its tributaries, including the South Fork Tolt River, have recently experienced high water temperatures above water-quality standards. Water managers can regulate flows to influence the temperature regime of the South Fork Tolt River downstream of the dam. More information is needed to determine how far downstream water temperature strategies are effective.
How USGS will help:
The USGS is studying the effects of temperature management strategies downstream of the South Fork Tolt Dam. We are analyzing water temperature data from the Tolt River Basin. We are also using numerical tools to simulate water temperature in the South Fork Tolt River.
Problem:
Throughout the Pacific Northwest, declining winter snowpacks, increased withdrawals of water from rivers and hydrologically connected aquifers, and changing land cover have contributed to increasing water temperatures in rivers. As a result, the frequency of water temperatures that are detrimental or lethal to different life-history phases of salmonids and other cold-water fish has been increasing in many rivers.
The Tolt River and its tributaries, including the South Fork (SF) Tolt River, have recently experienced high water temperatures in excess of water-quality standards, most notably during the summers of 2003 and 2015 when drought conditions prevailed. Regulation of flow in the SF Tolt River, however, affords water managers opportunities to influence the temperature regime of the SF Tolt River downstream of the dam by mixing water from intakes above and below the reservoir’s thermocline. To determine the longitudinal extent of the downstream influence of temperature-management strategies for the SF Tolt Dam, Seattle City Light needs analysis of existing temperature datasets collected within the Tolt River basin and numerical tools that simulate water temperature in SF Tolt River.
Objectives:
The objective of this study is to characterize the thermal regime of the Tolt River basin. We are evaluating the effects of currently implemented and future alternative operational strategies of SF Tolt Dam on downstream water temperatures. We are statistically analyzing existing water-temperature data and developing a water-temperature model of the SF Tolt River, which will be integrated with an existing model of the SF Tolt Reservoir.
Relevance and Benefits:
This study will provide water managers at Seattle City Light with the information needed to understand the effects of current and proposed SF Tolt Dam operational strategies on the Tolt River basin’s water-temperature regime and its consequences for its ecosystem. Additionally, the methods and tools developed for this investigation will be applicable for use in other regulated, salmon-bearing river systems throughout the western US.

Approach:
The USGS, in cooperation with Seattle City Light and the Tolt Fisheries Advisory Committee, will complete the following tasks:
- statistical analysis of existing water-temperature data in the Tolt River basin,
- development and calibration of a one-dimensional water temperature model of the SF Tolt River,
- numerical simulation of the downstream effect of SF Tolt Dam operational strategies on temperature in the SF Tolt River, and
- publication of water-temperature model simulation and report.
The USGS project team is listed below.
Nicholas A. Sutfin
Hydrologist and Surface Water Specialist
The Issue:
The Tolt River and its tributaries, including the South Fork Tolt River, have recently experienced high water temperatures above water-quality standards. Water managers can regulate flows to influence the temperature regime of the South Fork Tolt River downstream of the dam. More information is needed to determine how far downstream water temperature strategies are effective.
How USGS will help:
The USGS is studying the effects of temperature management strategies downstream of the South Fork Tolt Dam. We are analyzing water temperature data from the Tolt River Basin. We are also using numerical tools to simulate water temperature in the South Fork Tolt River.
Problem:
Throughout the Pacific Northwest, declining winter snowpacks, increased withdrawals of water from rivers and hydrologically connected aquifers, and changing land cover have contributed to increasing water temperatures in rivers. As a result, the frequency of water temperatures that are detrimental or lethal to different life-history phases of salmonids and other cold-water fish has been increasing in many rivers.
The Tolt River and its tributaries, including the South Fork (SF) Tolt River, have recently experienced high water temperatures in excess of water-quality standards, most notably during the summers of 2003 and 2015 when drought conditions prevailed. Regulation of flow in the SF Tolt River, however, affords water managers opportunities to influence the temperature regime of the SF Tolt River downstream of the dam by mixing water from intakes above and below the reservoir’s thermocline. To determine the longitudinal extent of the downstream influence of temperature-management strategies for the SF Tolt Dam, Seattle City Light needs analysis of existing temperature datasets collected within the Tolt River basin and numerical tools that simulate water temperature in SF Tolt River.
Objectives:
The objective of this study is to characterize the thermal regime of the Tolt River basin. We are evaluating the effects of currently implemented and future alternative operational strategies of SF Tolt Dam on downstream water temperatures. We are statistically analyzing existing water-temperature data and developing a water-temperature model of the SF Tolt River, which will be integrated with an existing model of the SF Tolt Reservoir.
Relevance and Benefits:
This study will provide water managers at Seattle City Light with the information needed to understand the effects of current and proposed SF Tolt Dam operational strategies on the Tolt River basin’s water-temperature regime and its consequences for its ecosystem. Additionally, the methods and tools developed for this investigation will be applicable for use in other regulated, salmon-bearing river systems throughout the western US.

Approach:
The USGS, in cooperation with Seattle City Light and the Tolt Fisheries Advisory Committee, will complete the following tasks:
- statistical analysis of existing water-temperature data in the Tolt River basin,
- development and calibration of a one-dimensional water temperature model of the SF Tolt River,
- numerical simulation of the downstream effect of SF Tolt Dam operational strategies on temperature in the SF Tolt River, and
- publication of water-temperature model simulation and report.
The USGS project team is listed below.