• 1. Total Dissolved Gas TMDLs
    1. Total Dissolved Gas TMDLs

    Hey there. I'm Salmo, a kokanee salmon. I was asked by the Lake Roosevelt Forum to tell you a little about Total Maximum Daily Load (TMDL) standards for total dissolved gas in Lake Roosevelt.

    TMDLs are part of setting water quality standards and implementing water improvement plans. These standards and plans let humans, fish and other species benefit from our precious rivers, lakes and streams. When I say benefits I mean using our waterways for things like drinking water, irrigation, recreation, power generation, flood control, and quality habitats for fish like me.

    With so many beneficial uses, it can be hard to balance everyone's needs and wants. To learn more about TMDLs, see my other TMDL slide shows in the Environment section of this website.

  • 2. What is TDG?
    2. What is TDG?

    Let's start by explaining what total dissolved gas (TDG) is. Basically, it's just air mixing with water. Technically, the air is dissolving into the water. Because you can't see the air, you don't see this mixing process. To create an image in your head, think about pouring sugar into water. Some of it mixes and dissolves into the water, and some drops to the bottom. How much becomes part of the water depends on different things you can do, like stirring the sugar in.

    When something causes air to dissolve into water, the water's nitrogen and oxygen content (which are the main properties of air) can go way up. This is called supersaturation.

  • 3. Gas Bubble Trauma
    3. Gas Bubble Trauma

    Particularly for fish species like me, this can cause health problems that can be as severe as dying.

    The problem is that when water is supersaturated with dissolved gas, the excess gas can be released into our blood stream and tissues, causing gas bubbles to form. That's why scientists often call this health problem gas bubble disease trauma.

  • 4. TDG Exposure Levels
    4. TDG Exposure Levels

    Depending on the level of supersaturation and the length of exposure, fish may experience very minor affects. For instance air bubbles in our fins may disappear in a day. On the other hand, long or high exposure can cause very severe bubble formation behind our eyes or in our gills, mouths and cardiovascular system. Secondary infections usually occur in these areas, which leads to death.

    Scientists know that fish experiencing TDG supersaturation of 115 to 130 percent can experience severe signs of gas bubble disease and high mortality. So to protect fish and other aquatic life, federal, state and tribal agencies have set water quality standards of 110% TDG saturation.

    In other words, if 100% saturation represents normal equilibrium with the atmosphere (meaning the same pressure in the water as in the air), the water quality standard says not to exceed "normal" by 10%.

    Photo shows gas bubble trauma with a secondary infection.

  • 5. What Causes TDG Supersaturation?
    5. What Causes TDG Supersaturation?

    So what causes TDG supersaturation? That's easy, think waterfalls.

    When water plunges over a waterfall and into a deep pool, it takes air bubbles with it. The high pressure causes the bubbles to change into a solution. This mixes with the normal water, so now the water is supersaturated with gases, primarily nitrogen (70%). It's kind of like putting the fizz into soda pop.

    So if you're looking at a beautiful waterfall, you may be seeing TDG supersaturation happening as a normal part of nature. And if you're looking at water falling over the spillway of a dam, you may be seeing it as part of what humans create.

    In the Columbia Basin, there are a lot of dams in both Canada and the United States. As a result, monitoring and finding ways to reduce TDG supersaturation to safe levels is a big deal.

    Photo shows McNary Dam.

  • 6. TDG and Lake Roosevelt
    6. TDG and Lake Roosevelt

    So let's take a look at total dissolved gas and Lake Roosevelt. The Columbia River originates in Canada and flows into Lake Roosevelt at the border. On the Canadian side, there are four large dams and six smaller dams that are either on the Columbia or feed into the Columbia.

    In addition, the Pend Oreille River flows from the United States into the Canadian portion of the Columbia. As a result, dams on and upstream of the Pend Oreille River also contribute to TDG that comes into Lake Roosevelt.

    The net result is that water flowing across the border into Lake Roosevelt exceeds the 110% TDG water quality standard about five to six months of the year.

    The Spokane River also feeds into the southern part of Lake Roosevelt. There are six relatively small dams on this stretch of the river, with Little Falls being the closest to Lake Roosevelt.

    Photo shows Brilliant Dam, Canada.

  • 7. When Does Supersaturation Occur?
    7. When Does Supersaturation Occur?

    TDG supersaturation often happens in the spring when melting snow pack creates heavy flows and/or flooding. Water that can not be stored in the reservoir behind a dam or passed through turbines to generate electricity is sent over the spillway. TDG supersaturation occurs as the water plunges into the basin below.

    Hydroelectric operators also spill water to help downstream salmon migrate to the ocean. And sometimes spill occurs because maintenance within a powerhouse forces operators to send water over a spillway. So while TDG supersaturation is most common in the spring time, it can happen during other seasons as well.

    Along the Columbia, including Lake Roosevelt, an extreme case of TDG supersaturation occurred in 1997. This was a year when we had lots of snow pack that melted quickly in late April. In both Canada and the United States, this forced dam operators to open their spill gates and allow the water to come rushing through. TDG supersaturation soared, killing thousands of fish.

  • 8. Surface Exposure
    8. Surface Exposure

    Fish swimming closer to the surface had the highest rate of injury or mortality. Let's explore why.

    When TDG is at 110% at the surface of a water body, the TDG saturation one meter (3 feet) below the surface is 100%. For each meter you go down in the water, because the water pressure goes up, the TDG saturation actually experienced by the fish drops by 10%. So when the surface TDG is 120%, a fish two meters down is exposed to a TDG level of only 100%

  • 9. Hatcheries & Net Pens
    9. Hatcheries & Net Pens

    In Lake Roosevelt, over one million rainbow trout and kokanee are raised in hatcheries and net pens each year. Fish in net pens are generally released around the end of May, which is when they are large and strong enough to survive on their own.

    The net pens float on the surface of the water. Within the net pen, fish travel anywhere from the surface to a depth of up to fourteen feet (4.3 meters). Being held in this small area means they are particularly vulnerable to TDG supersaturation.

    Photo shows net pens on Lake Roosevelt.

  • 10. What Scientists Have Found
    10. What Scientists Have Found

    Before talking about what's being done to reduce TDG, let's quickly review what scientists have found over the years.

    Scientists know that the closer to the surface fish travel or are held, the more vulnerable they are. Scientists also know that when TDG travels through slow, flat waters (like Lake Roosevelt), it tends to stay in the water. But if the water moves across rapids or other high turbulent conditions, excess TDG is reduced because the gas makes contact with, and is released, into the air. So one of the things being monitored is how long it takes for the TDG to dissipate (be released) after being created at various projects.

    Lastly, scientists know that TDG is minimized if dams that generate electricity maximize how much water flows through the powerhouse and turbines rather than over the spillway. By water passing through the powerhouse, there is no waterfall. That's less pretty to the human eye but it creates healthier water conditions for me.

  • 11. What's Being Done?
    11. What's Being Done?

    With these dynamics in mind, numerous federal, state, and tribal agencies in the U.S. have been working with their counterparts in Canada to reduce TDG levels. And I'm happy to say, these actions are helping out. In fact, in 2003 several improvements on Canadian projects will start to take effect.

    At some Canadian and U.S. dams, additional powerhouses and turbines are being built. This allows more water to pass through the powerhouse rather than over a spillway. A nice side benefit is that these improvements can generate additional and needed electricity for homes and businesses.

    Similarly, there are dams where managers are running the water through existing powerhouses even when there isn't a demand for electricity.

    Structures are also being added to dams to "strip" some TDG out of the water. Spill deflectors (also called flip lips) are examples of such structures. And lastly, hydroelectric operators have found that sometimes changing which gates water flows through helps.

    The transboundary gas group has an excellent web site for finding out what actions are taking place, and where.

  • 12. TDG Supersaturation Elsewhere
    12. TDG Supersaturation Elsewhere

    TDG supersaturation, of course, is not just a Lake Roosevelt issue. The water quality standard for TDG is not being met below dams along the length of the Columbia and Snake rivers. This is why these rivers were put on the list of impaired water bodies (also called the 303(d) list), and a total dissolved gas TMDL is being developed for each.

    The Washington Department of Ecology is taking the lead on developing the TMDL for state waters from Grand Coulee Dam down to the Snake River and the EPA is taking the lead, working closely with the Colville Confederated Tribe and the Spokane Tribe, on Lake Roosevelt and Tribal waters below Grand Coulee Dam.

    The TMDL will allocate how much TDG can be present above and below each project and still meet the water quality standards. Visit the EPA TDG site for details.

  • 13. TMDL Development & Implementation
    13. TMDL Development & Implementation

    A TMDL for TDG has been issued for the Lower Columbia River, and a TDG TMDL for the Lower Snake River will be issued soon.

    A draft TMDL for the Mid-Columbia and Lake Roosevelt is currently being reviewed and considered. Before the TMDL is issued, implementation strategies will be developed in a collaborative forum with all interested parties to identify what actions will be taken and by whom.

  • Thanks!

    Well, thanks for staying with me! I hope this has been helpful. If you'd like to learn more about TDG, the TMDLs Resource Links page may be helpful, and don't miss the other TMDL slideshows in the Environment section of this website.