One of the factors jeopardizing salmon passage through the Snake and Columbia Rivers is the elevated level of total dissolved gas (TDG) dams produce as they spill water. TDG can cause gas bubble disease in fish and ultimately be lethal. As a part of the U.S. Army Corps of Engineers "Gas Abatement Study", Waterways Experiment Station (WES) has developed equations to predict tailwater gas production as a function of spill. (The tailwater of a dam is the water just downstream.) These equations were implemented in the Columbia River Salmon Passage model version 1.6. The implementation is part of a larger effort to improve TDG modeling to better access the impacts of supersaturation on migrating salmon and predict the potential benefits of systemwide reduction of TDG.
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Development of the TDG Model
The Columbia River Salmon Passage Model (CRiSP) produces estimates for total dissolved gas (TDG) in the Snake and Columbia Rivers. The primary input of TDG is from spill at the dams. The equations used for gas production from spill include the equations developed by Army Corps of Engineers Waterways Experiment Station (WES) in 1996 and 1997 as well as additional work done by Columbia Basin Research. For equation details, see the II.5 Total Dissolved Gas section of the CRiSP1.6 Theory & Calibration Manual.
Model results for the years 1995-1997 look good. They can be viewed by dam or by year: 1997, 1996, 1995. Several changing conditions at specific dams in 1998 made it necessary to refit the gas models at a couple dams. One such dam is Ice Harbor, where deflectors were added to four inside bays before the 1997 season and 2 spillbays were closed as well for the 1997 season due to construction. A new equation was consequently fit for Ice Harbor that year. Similary, for 1998 with the 2 more deflectors in operation the Ice Harbor gas dynamic was refit to current tailwater and spill data. John Day was also given a new gas dynamic because of several flow deflectors that are new as of the 1998 season.
The tailwater gas predictions are for the spill side of the river. For Bonneville, The Dalles and Dworshak, the powerhouse and spill flows are instantaneously mixed, as is suggested by the data. For the tailwater estimates of the other dams, the maximum of the incoming forebay level and the modeled amount of spill-generated TDG is used. Thus during periods of low spill and elevated forebay TDG levels, the tailwater estimate is essentially the forebay estimate. This generally happens only for levels of supersaturation <110%. Due to a large amount of mixing happening below Bonneville and The Dalles, the modeled gas levels for these projects are the *mixed* TDG levels from modeled powerhouse and spill flows.
Observed TDG Data
The observed data are provided as a courtesy by the U.S. Army Corps of Engineers from the fixed monitors below the dams. Quality assurance may not be complete when the data is first provided. Sudden dips in observed TDG data during periods of high spill are indicators of suspicious data. Corrected data are displayed where possible. The current U.S. Army Corps of Engineers Total Dissolved Gas reports can be consulted for reference. For interactive access to water quality data and other pertinent river data, see the data report tool Columbia River DART.
Where TDG is Modeled
The fixed monitors are usually about 1 mile below the dam (Dissolved Gas Monitoring Network map). The forecasted gas production shown predicts the gas observed by these monitors. Gas levels in the stilling basin have been observed to be 20-30% higher. It should also be noted that the nearest downstream monitors to Bonneville Dam were 6 miles downstream, so it is expected that the gas levels at these monitors (WRNO and SKAW) will be lower than those generated at the dam.
Spill and Flow Data
The accuracy of gas forecasts for the coming year depend on the accuracy of the data coming from the predicted flows and planned spill from the most recent flow forecast provided to Columbia Basin Research by the Bonneville Power Administration.