You may have noticed that the water of the Brazos River and its tributaries seems to be cloudier and more discolored, and you also may have wondered what is causing this.
The discoloration is called turbidity — the amount of suspended particles, dissolved minerals and dissolved organic matter in water. It is the factor that causes water to appear cloudy or even opaque. It is measured by how the particles scatter light rays moving through the water.
The more suspended solids there are in the water, the murkier the water appears. With the recent storms that drenched the state in May and June, plenty of nutrient- and sediment-laden runoff has entered the river basin, increasing the suspended sediment and algal concentrations of area waters and thus increasing the turbidity.
The U.S. Environmental Protection Agency notes that these suspended materials in the water consist of soil particles (clay, silt, and sand), algae, plankton, microbes, and other substances.
High turbidity can affect the habitat quality of the aquatic ecosystem, and recreational quality. Higher turbidity can increase water temperatures, in slow moving waters, because suspended particles absorb more heat. Elevated water temperatures, in turn, can reduce the concentration of dissolved oxygen (DO). Turbidity reduces the amount of light penetrating the water, which reduces photosynthesis and can reduce primary production depending on the waterbody. As the particles settle, they can blanket the stream bottom, especially in slower waters, and damage the habitat for fish, benthic macroinvertebrates, and aquatic vegetation.
Sources of turbidity may include:
- Soil erosion
- Waste discharge
- Urban runoff
- Eroding stream banks
- Large numbers of bottom feeders (such as carp), which stir up bottom sediments
- Excessive algal growth
- Land disturbance/development.
According to the United States Geological Survey, “During periods of low flow (base flow), many rivers are a clear green color, and turbidities are low. During a rainstorm, particles from the surrounding land are washed into the river making the water a muddy brown color, indicating water that has higher turbidity values. Also, during high flows, water velocities are faster and water volumes are higher, which can more easily stir up and suspend material from the stream bed, causing higher turbidities.”
So how long will the high turbidity remain in the Brazos Basin? That depends on a variety of factors, including sub-watershed location, and whether or not additional rainfall results in runoff.
A certain amount of turbidity is to be expected and is normal for all rivers. Some rivers in the Brazos Basin are naturally more turbid than others due to their underlying geology, regardless of rainfall and runoff.
It may take several weeks to several months depending on location, but the turbidity in the Brazos will settle. While waiting for the waters to return to normal conditions keep in mind that high turbidity can impair navigation by reducing visibility in the water.