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Biological Effects of TDG Supersaturation

by Army Corps of Engineers
Dissolved Gas Abatement Study, May 2002


The effect of TDG supersaturation on salmonids is very complex and depends upon several factors including the level of TDG supersaturation, amount of exposure time, water temperature, physical condition of the fish, and swimming depth of the fish.

All of these factors contribute to the conditions under which fish may be exposed to uncompensated levels of TDG. It is exposure to uncompensated TDG that may result in GBT symptoms and physiological damage. Exposure time to uncompensated TDG is a function of migrant behavior and the time and spatial dependent distribution of TDG.

Uncompensated TDG supersaturation is that portion of total gas pressure in excess of atmospheric level that is not compensated by hydrostatic pressure at the depth of the fish. Approximately 10 percent of total gas pressure above 100 percent (atmospheric level) is compensated by each 3.28 feet of depth. For example, a fish acclimated to a TDG pressure of 120 percent will not experience a gradient across its membranes and other tissues at depths of 6.56 feet or greater. Therefore, the physical conditions will not be present that would permit dissolved gas to come out of solution and begin the formation of bubbles.

Fish exposed to high total gas pressure (TGP) exhibit a variety of physiological signs that are harmful or fatal. As a class, these signs are referred to as gas bubble trauma (GBT) or gas bubble disease. The major signs of GBT that can cause death or severe stress in fish are:

Other signs of GBT include exopthalmia and ocular lesions, bubbles in the intestinal tract, loss of swimming ability, altered blood chemistry, and reduced growth. Individually or in combinations, these signs may compromise the survival of fish exposed to elevated TGP over extended periods.

Each sign of GBT involves the growth of gas bubbles internal and/or external to the fish. However, for each sign, a threshold in delta pressure must be exceeded before bubble formation or swim bladder overinflation can begin. Still, in some cases, the activation of GBT signs is not an easily demonstrated cause-and-effect relationship.

This difficulty is due to bubbles that develop internally to the fish that may form in many body compartments, disrupting neurological, cardiovascular, respiratory, osmoregulatory, and other physiological functions. Thus, depending on the TGP and DP, multiple signs may be present in fish. The GBT may also increase the susceptibility of aquatic organisms to other stresses, such as bacterial, viral, and fungal infections. The GBT weakens fish, especially juvenile life stages, thereby, increasing their susceptibility to predation. TDG supersaturation and GBT may also affect the food chain of an aquatic ecosystem. For example, TDG supersaturation or GBT may increase or decrease the availability of a food source for a particular species. The result may be the species increases in abundance or becomes locally extinct. Given this background information, it is clear that GBT mortality may result from a variety of direct and indirect effects of TDG supersaturation.

The analysis of risk of physiological injury resulting from exposure to TDG supersaturation considers the exposure history of migrants during the whole of their passage through the federal hydropower system. The reason is that, unlike processes responsible for physical injury that are specific to a particular location, TDG supersaturation, while generated at dams, is propagated throughout the system and constitutes a threat to fish health considerably beyond the point of generation. In addition, downstream transport and mixing processes for TDG supersaturation vary with each project. These variations translate into differences in the downstream environment from project to project. The convolution of the environmental consequences of these physical processes with project-to-project variation in fish passage behavior results in differences in fish health risk between projects with equivalent reductions in TDG supersaturation production. Therefore, the benefit to migrant health of DGAS alternatives cannot be assessed without considering the whole of the processes constituting the threat to fish health. In the case of TDG supersaturation, this includes the consequences of the transport and dispersion of dissolved gas throughout the hydropower system.

The health risk of TDG exposure drops considerably as the water quality standard is approached. This results from the combined effect of two factors. The first is reduced risk of GBT upon exposure to TDG supersaturation conditions and the second is concurrent reduction in the portion of the fish's habitat at "effective" TDG levels above 110 percent. For example, at 120 percent TDG, only fish located in the upper 3.28 feet, or less, of the water column for extended periods are at any risk from GBT. In addition, the likelihood of developing GBT at TDG levels less than 120 percent appears to be low. The combination of these factors results in the risk to migrating juvenile salmonids of GBT from exposure to TDG supersaturation conditions likely being very small at TDG supersaturation levels within 10 percent and perhaps as much as 15 to 20 percent of water quality standards. Because of direct linkage between risk and benefit, the low risk of GBT at these levels would also mean that the incremental benefits of gas abatement alternatives are also likely to be small. The initial DGAS plan of study contained analysis of DGAS alternatives to evaluate the incremental benefits of reductions in TDG supersaturation as water quality conditions approached standards for TDG. These analyses were to include consideration of the financial and biological consequences of implementation. Initial engineering analyses have indicated that once flow deflectors are installed at federal projects, the costs of incremental gains in gas reduction are likely to be high. The biological consequences of implementation include, but are not limited to, the potential for physical injury during passage through a gas abatement structure. It is not obvious at this time that the tradeoff between incremental reductions in TDG (beyond those achievable with flow deflectors), and the benefits to fish health provided will offset impacts to fish health during passage through gas abatement structures for some gas abatement alternatives.

Recent studies completed at the Department of Energy's PNNL have shown mortality of juvenile salmonids and bluegills acclimated to TDG from 120 percent to 135 percent at a pressure of 15 psi gauge (equivalent to a depth of approximately 33 feet) then exposed to a pressure cycle typical of that juvenile fish experience when passed through hydroturbines in the FCRPS (personal communication, Scott Abernethy, PNNL, July 2000). Necropsies performed on fish that died as a result of this exposure scenario showed ruptured swim bladders and bubbles in the heart, gills, and other internal organs. Injuries observed were typical for fish exposed to uncompensated levels of TDG supersaturation. This finding, if substantiated, could have implications for management of TDG within the FCRPS.


The Federal Clean Water Act has established water quality standards for TDG in surface waters to be 110 percent of saturation with respect to the local barometric pressure. The local States have developed more specific and detailed descriptions of the TDG water quality standard and generally provide oversight of compliance in relation to protection of the natural resource.

The Washington State Department of Ecology TDG water quality standard for surface waters states "TDG shall not exceed one hundred ten percent (110 percent) of saturation at any point of sample collection" (Chapter 173.201A-030 WAC, Water Quality Standards for Surface Waters of the State of Washington). It also states, "When spilling water at dams is necessary to aid fish passage, total dissolved gas must not exceed an average of one hundred fifteen percent (115 percent) of saturation as measured at Camas/Washougal below Bonneville or as measured in the forebays of the next downstream dams. The TDG concentrations must also not exceed an average of one hundred twenty percent (120 percent) as measured in the tailraces of each dam. These averages are based on the twelve highest hourly readings in any one-day (24-hour period) of TDG. In addition, there is a maximum TDG concentration one hour average of one hundred twenty-five percent (125 percent), relative to atmospheric pressure, during spillage for fish passage." This exemption/adjustment is also for periods exceeding powerhouse capacity up to the 7-day, 10-year frequency flows (Chapter 173.201A-060 WAC, Water Quality Standards for Surface Waters of the State of Washington).

The Oregon Department of Environmental Quality (DEQ) states the concentration of TDG relative to atmospheric pressure at the point of sample collection shall not exceed 110 percent of saturation, except when stream flow exceeds the 7-day, 10-year frequency flow or average flood. However, for hatchery receiving waters and waters of less than 2 feet in depth, the concentration of TDG relative to atmospheric pressure at the point of sample collection shall not exceed 105 percent of saturation. The DEQ allows a TDG waiver for fish spill season. This waiver allows up to 120 percent at the tailrace/tailwater TDG monitor or up to 115 percent in the downstream forebay or at the Camas monitor below Bonneville. This measure is calculated as the calendar day average of the highest 12 hourly readings. This calculated statistic is not to exceed the waiver standard. There is a maximum allowable 2-hour average of 125 percent, relative to atmospheric pressure.


. . .


The MASS1 and MASS2 numerical models (appendix H) for hydrodynamics, energy, and dissolved gas transport were used to compare the performance of different gas abatement system alternatives. The system alternatives analyzed were primarily structural modifications to the dams, but operational changes such as the use of uniform spill patterns were also included. These alternatives are summarized in tables 9-2 and 9-3 in section 9. Seven potential system alternatives were compared against a system baseline utilizing the numerical models coupled with predictive gas production equations identified in appendix G. The system alternatives were grouped as either short-term alternatives or long-term alternatives. The short-term alternatives are those that are considered to be of conventional design, with proven benefits less expensive and that can be implemented more quickly than long-term alternatives. The system alternatives were compared to a system baseline utilizing performance factors of: (1) TDG loading, (2) habitat impacts, and (3) water quality standards for TDG. A summary of model-derived observations are discussed in the following paragraphs.

a. Summary Short-Term Alternative Analysis.

For the short-term alternatives (system alternatives 2 through 5, system alternative 1 is baseline), the added deflectors appear to yield the most improvements in TDG water quality parameters on the Columbia River projects whereas the addition of powerhouse/spillway separation walls are forecasted to be the most beneficial for lower Snake River projects. This indicates the current fast-track program for installing deflectors will probably result in the quickest and most reliable reduction in TDG pressures throughout the Columbia River system. The addition of flow deflectors at Lower Monumental and Little Goose on the Snake River will also result in tangible reductions in TDG exchange for comparable spill volumes. However, the implementation of powerhouse/spillway separation walls at these projects would yield even larger reductions in the TDG loading throughout the Snake River. This corresponds to system alternatives 3 and 4.

System alternative 3 includes: (1) added deflectors at Little Goose, Lower Monumental, McNary, John Day, and Bonneville and (2) changing spill patterns from existing patterns to more uniform patterns at Little Goose, Lower Monumental, McNary, and Bonneville. System alternative 4 includes those features and operations proposed by system alternative 3 plus: (1) a uniform spill pattern at Lower Granite, John Day, and The Dalles; (2) new spillway deflectors at The Dalles; and (3) powerhouse/spillway separation walls at Lower Granite, Lower Monumental, Little Goose, and McNary. Utilizing the TDG loading evaluation criteria, the implementation of alternative 4 was shown to decrease the system TDG loading by approximately 60 percent (when compared to the baseline). It has also shown an increase in the aquatic habitat (for either depth compensated or uncompensated analyses) by volume in the receiving waters by approximately 60 percent. And, utilizing the water quality standards as criteria, approximately a two-thirds reduction in the water quality criteria exceedences would be realized in conjunction with system alternatives 3 and 4. It was assumed that the estimated realized benefits of the total dissolved gas abatement alternatives would be associated with the historic operations experienced during the study years.

b. Summary Long-Term Alternative Analysis.

Long-term system alternatives 6, 7, and 8 were also modeled with the MASS1 and MASS2 numerical models. System alternative 8 was the most successful at achieving water quality standards and in minimizing downstream TDG loading. For the hydrologic conditions simulated, over 90 percent of the water quality standard exceedences were eliminated with alternative 8 (when compared to the baseline). This alternative consists of: (1) the presence of deflectors on all spillway bays for each of the eight Snake River and Columbia River Corps projects; (2) uniform spill patterns implemented at each of the eight projects; (3) powerhouse/spillway separation walls installed at Lower Granite, Little Goose, and Lower Monumental; (4) new spillway structures (nine bays) installed at Lower Granite, Little Goose, and McNary; (5) new spillway structure (six bays) installed at John Day; and (6) submerged spillway gates at Bonneville.

c. Project-Specific Observations.

The evaluations of project-specific features influencing TDG exchange were derived from physical model studies, field studies, and numerical model simulations. From the system-wide perspective, certain trends or response patterns have emerged that can be used to characterize major reaches but, in general, there is a high degree of variation in forecasted responses between the lower Snake River and the Columbia River projects. This trend is at least partly due to the high spill ratios for Ice Harbor and the confluence of the Middle Columbia and lower Snake Rivers in the McNary pool.

The open river reach below Bonneville is much shallower than the ponded river reaches throughout the remainder of the study area, resulting in a higher percentage of the habitat residing above the compensation depth for comparable TDG conditions.

The numerical simulations project that the addition of deflectors at Bonneville and at The Dalles will result in significant improvements in TDG in the tidal pool downstream from Bonneville. The addition of submerged spillway gates at Bonneville (alternative 8) shows the greatest potential for improvement.

Simulations of the operation of an additional six-bay spillway at John Day (alternative 6) and a nine-bay spillway at McNary (alternative 7) produced significant improvements in water quality below these two dams. At McNary, the greatest improvements in TDG were associated with the combination of uniform spill patterns, additional spillway deflectors, a powerhouse/spillway separation wall plus the addition of a nine-bay spillway.

Very little change at Ice Harbor is observed from the simulations for any but system alternative 8. Ice Harbor has very effective flow deflectors in place at the project. This, coupled with a high ratio of spill to powerhouse discharge, tends to reset or re-establish TDG conditions for the Snake River to reflect Ice Harbor conditions and operations. These conditions simulated in the model effectively overrode any major TDG impacts from the upstream projects on the Snake River and may be responsible for the noticeable difference in forecasted responses between the two rivers. The addition of a powerhouse/spillway separation wall resulted in limited improvements in TDG at Ice Harbor.

The three upper river projects (Lower Monumental, Little Goose, and Lower Granite) all respond similarly and favorably to the addition of a powerhouse/spillway separation wall. As would be expected, additional spillways with nine spillway bays each at these same three projects result in significant improvements in downstream water quality. Modest water quality benefits are also forecasted from the deflector additions at Lower Monumental and Little Goose when compared to the existing spillway operation utilizing only six bays with flow deflectors.

It should be noted that the conclusions above only consider the potential water quality benefits associated with the various gas abatement alternatives. Final decisions or recommendations should also incorporate potential impacts on migration of anadromous fishes (both juvenile and adults), other resident biological communities, power production, navigation, and any of the other intended uses of projects on the Columbia and lower Snake Rivers.


If a decision is made to pursue TDG reduction measures beyond the current fast-track deflector optimization program, then the following information is offered for consideration with one caveat--physical injury induced by alternative gas abatement devices should be evaluated (and relative risk compared to gas supersaturation in the river-reservoir system and other fish passage approaches) before novel devices are installed.

a. Suggested Implementation Process.

  1. Additional Deflectors.

    Installation of additional deflectors is currently scheduled for Bonneville and McNary in Fiscal Year 2002, Lower Monumental and Little Goose in Fiscal Year 2004, and The Dalles in Fiscal Year 2005 as a part of the current deflector optimization program.

  2. Powerhouse/Spillway Separation Wall. Installation of the powerhouse/spillway separation walls would require the following process with the anticipated durations once decision documents are completed and appropriations are obtained. The first powerhouse/spillway separation wall would be installed on a single project such as Lower Granite. Following construction, a TDG and biological evaluation would be conducted to assess the separation wall performance. If successful, the benefits might then justify installation at the other three lower Snake River projects as well as the McNary and John Day projects on the Columbia River. Duration and process steps are identified in table 11-1.

    If deflectors and powerhouse/spillway separation walls are installed, then a reassessment of baseline TDG performance would be recommended before pursuing additional structural changes.

    The installation of additional spillway structures on Lower Granite, Little Goose, Lower Monumental, Ice Harbor, McNary, and John Day and installation of submerged spillway gates on Bonneville would likely follow the following sequence. Once decision documents and appropriations are received: (1) design and build the modifications to the first selected dam; (2) conduct post-construction evaluations both biologically and physically to achieve a proof of concept; and (3) design and build modifications to the remaining dams. This process could take 20 to 30 years if each dam were to be modified in a sequential process. However, if funds can be made available, this could be reduced to possibly a 15- to 20-year process by design and construction of modifications to more than one dam simultaneously.

b. Approximate Range in System Costs.

The system costs (see tables 11-2 and 11-3) have been estimated for three optional levels of total dissolved gas improvement. These costs include overhead, profit, construction bond, planning, engineering, design, construction supervision, administration, and contingencies.

Table 11-2
Approximate Range in System Costs - Lower Snake Projects

Project Option - 1
Deflector Only
Option - 2
Deflectors Plus
Powerhouse Spillway
Separation Walls
Option - 3
Deflectors Plus
Powerhouse Spillway
Separation Walls
Plus 9 Spillway Bays
Low High Low High Low High
Lower Granite $0 $0 $19 $31 $295 $472
Little Goose $8 $8 $27 $39 $307 $487
Lower Monumental $10 $10 $29 $41 $442 $701
Ice Harbor $0 $0 $19 $31 $331 $530
Total Lower
Snake River
System Costs
$18 $18 $94 $142 $1,375 $2,190

c. Order of Implementation.

Selection of alternatives for implementation and the associated priority of implementation depend on many factors. These factors include water quality criteria, fish passage, construction costs, dam safety, navigation, operation and maintenance costs, and construction scheduling.

The modeling results show that the Snake and Columbia Rivers are not strongly coupled (given current operations of spill levels at Ice Harbor) and the alternative implementation priority (Snake River projects versus Columbia River projects) could be selected independently, provided that gas abatement is the primary criteria. Furthermore, if peak TDG concentrations are used as the principal criteria, the selection could be based only on the performance in an individual reservoir.

If impacts to habitat are the criteria used for decisions about implementation priority, then projects such as McNary and Bonneville have added significance due to the extremely large volumes and reaches of the receiving waters. Abatement at these projects will affect larger volumes of actual aquatic habitat and larger proportions of the river system. The results clearly show that alternatives that reduce TDG concentrations at The Dalles and Bonneville will lead to proportional TDG reductions in the Columbia River below Bonneville. This indicates that a possible implementation schedule would prioritize these two projects in order to realize the benefits in the river below Bonneville. Reducing TDG in the river below Bonneville could be significant because more of that available habitat is less than 6.56 feet deep, which means there is less opportunity for fish and other aquatic species to avoid GBT by being below the compensation depth. In addition, every salmon smolt must pass through the estuary during its out-migration.

The addition of a powerhouse/spillway separation wall at Lower Granite would allow most the powerhouse flows to dilute the high TDG pressures generated during spillway operation. The consistently low forebay TDG pressures at Lower Granite should be an important consideration in scheduling post-deflector gas abatement measures in the Snake River. The high rates of powerhouse entrainment observed at Little Goose and potential reduction in TDG loading associated with partitioning these waters with a separation wall should provide a strong incentive for this alternative.

Related Sites:
Dissolved Gas Abatement Study - Phase II Final Index Page, Army Corps of Engineers, May 2002
Total Maximum Daily Load for Lower Snake River Total Dissolved Gas by Paul J. Pickett and Mike Herold, WA State Department of Ecology, August 2003

Army Corps of Engineers
Biological Effects of TDG Supersaturation
Dissolved Gas Abatement Study, May 2002

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