Betty’s Branch Fish Kill
By: Oscar Flite, Ph.D.
I use my most distilled explanation of the water cycle for my four-year old son, it goes like this:
The rain falls on the creek, the creek flows to the river, the river flows to the ocean, the ocean makes the clouds, the clouds make the rain, the rain falls on the creek…
The surface of the ocean is the lowest surface-water level on earth (sea level). Since water flows downhill, it makes sense that all freshwater flows toward the ocean. Freshwater primarily flows toward the ocean in channels we call rivers and streams. While the ocean is the lowest water elevation on the surface of the earth, rivers are the second lowest water elevation on the surface of the earth; rivers flow toward the ocean. If the elevation of a river increases (because of higher river flows), it can have a dramatic impact on the land surrounding the river, especially if the land is the same elevation as the rising river; this we know as flooding. When the river elevation rises, it has an immediate effect on the smaller creeks that flow toward the river. Since water flows downhill and the river elevation is rising, the creek water can no longer flow toward the river and the creek water starts to rise too. Water that gets backed up in creeks starts to flood into land areas that do not normally undergo such wet conditions. Those areas are often highly vegetated and range from wetlands to nicely manicured backyards. If the river and creeks remain elevated for a few days, bacteria in the water and soil will start to decompose the newly wetted vegetation and within several days (depending upon the season) the bacteria will start using the newly flooded vegetation for food. That bacterial consumption has a direct impact on the amount of oxygen in water. Much like when you and I eat vegetables, our bodies convert that plant material into carbohydrates and water, but it is done at the expense of oxygen in in our blood; our bodies need oxygen to convert the plant material to carbohydrates for energy. So too do the bacteria in the water. However, unlike the large and nearly limitless amount of oxygen in the atmosphere, the water has a more limited amount of oxygen. If the oxygen in the water is used up at a rate higher than it can be produced by algae or replaced by wind mixing, then the dissolved oxygen concentration in the water will decrease which can ultimately lead to suffocation of animals that require dissolved oxygen to be above a certain concentration.
A fisherman observed a large fish kill in Betty’s Branch, a creek that flows to the Savannah River, on July 27, 2013. Preceding the fish kill, flows within the Savannah River were unusually high as a result of a nearly month-long rain event starting in late June 2013 (see Figure 1). Under “normal” flows throughout the year, the river gauge near the Fury’s Ferry bridge reads an average of 14 feet. However, during the flood event the water increased over 5 feet according to the gauge and remained at that level for nearly 2 weeks. On July 19, 2013, flows in the river were rapidly reduced by nearly 2 feet to an average gauge height of nearly 18 feet; flows remained there for nearly 1 week. On the morning of July 27, 2013, the morning of the fish kill, the river flows were rapidly reduced by nearly 3 feet to a gauge height of 15 feet. The extended flooding condition within the vegetated areas of Betty’s Branch was long enough for the bacteria to decay vegetation and use up all the dissolved oxygen in the stagnant water. The rapid decrease of river elevation caused a release of low dissolved oxygen water from the stagnant pools within the wetlands and backyards to flow toward the creek, ultimately suffocating fish within the creek. After the fish kill, GADNR contacted SNSA to discuss the fish kill. During that time, one of the biologists said they measured high concentrations of iron in the water. This piece of data was strong evidence that the rapid decrease of river level caused low dissolved oxygen water to rapidly flow to the creek because once bacteria use all of the available dissolved oxygen within the water, other bacteria will use nitrate, then iron and manganese, then sulfate, and finally carbon dioxide for energy. This series of energy producing substances for bacteria is called the “respiration cascade” and is a well-known series of chemical reactions that allow bacteria to inhabit almost every environment known. In essence, the measured iron played an important role in allowing us to understand the complex scenario of this particular fish kill; the iron severed as an indicator of the water quality and water flow conditions.
While many fish were killed during this event, the positive aspect of this event is that it easily allows us to recommend operational strategies to avoid killing fish under similar circumstances in the future. Since the USACE regulates flows within the Savannah River, the rapid decreases in river flow were caused by decreased water discharge from Thurmond Dam. In the future, flows after extended flooding could be slowly ramped down over several days to allow for slow release of low dissolved oxygen water from flooded areas. This would slowly introduce smaller amounts of low dissolved oxygen water to the creek, which will either give fish an opportunity to seek higher dissolved oxygen water or dilute the low dissolved oxygen water with more oxygenated water in the creek. Over the next few years, the USACE will be seeking information and data on how to better operate flows within the river with one of the main goals of protecting the river ecosystem. Lowering the flows after flooding will be one easy operational recommendation for ecosystem protection.