By Scott Bryan, CAP Senior Biologist
Numerous studies have generally described the water quality in the Lower Colorado River as “good,” and Central Arizona Project (CAP) takes pride in preserving and protecting that status for our customers.
When quagga mussels were found in Colorado River reservoirs in 2008, CAP knew that there would be negative impacts to manage. The obvious negative impact is the biofouling problems that result from the mussels attaching to critical equipment. But one of the issues that most people don’t consider is the impacts on water quality. Quagga mussels have an amazing ability to filter large amounts of water; a single mussel can filter one liter (0.25 gallons) of water each day. Considering that quagga mussels number in the trillions in Lake Havasu, you can imagine how quickly they can filter the entire volume of the reservoir. Many recreationalists see this as a benefit of the mussels because water clarity increases dramatically. But what many people don’t realize is that quagga mussels only filter out the “good stuff,” like zooplankton and phytoplankton. They tend to leave behind contaminants and they excrete large amounts of nutrients. The clear water and excess nutrients create perfect conditions for the growth of aquatic vegetation, which chokes water flow and threatens CAP’s ability to efficiently deliver water throughout the summer months.
The weeds are growing at an unprecedented rate in the reservoir. We start to see weeds within CAP’s 65-acre intake channel in April each year. In July, the weeds reach the end of their growth cycle and begin to die. When the weeds die, they often uproot or break off and float to the water surface. The dead vegetation then forms large mats that will move around the lake based on wind, weather, and water current. These large mats have the potential to become entrained in the flow created by CAP pump units and can restrict water flow at the pump intakes.
Since 2009, CAP has utilized a number of methods to ensure that we can maintain our ability to efficiently deliver water to our customers. These methods include mechanical removal with weed harvesting boats, excavators, and trash rakes, as well as chemical control to minimize weed growth. Although these methods do not provide a singular solution to the weed problem at Lake Havasu, used in combination, they provide a reliable manner in which to ensure that water continues to flow into the CAP canal.
In addition to vegetation suppression and weed removal techniques, CAP began using advanced sonar technology in 2012 to gain a better understanding of patterns in weed growth within the lake. A boat-mounted Lowrance Chartplotter is used to map the contours of the intake channel as well as gather valuable weed growth information. The monthly data collected with the chartplotter is uploaded to a cloud-based processing service to create bathymetric and weed density maps.
From the data and weed coverage maps, we have been able to pinpoint where and when our greatest challenges lie. For example, the data helps us to better plan for weed removal activities and determine the timing of peak weed mat formation. Also, maps are used to identify areas where application of herbicides would be most effective. Finally, we have been able to build a long-term data-set (6-year) that shows us the trends and cycles of weed growth that gives us better predictive capabilities. This helps us to effectively plan for weed removal earlier in the year so that personnel and equipment can be effectively scheduled. Overall, the data collection and mapping help us to ensure the reliability of CAP to withdraw and deliver water as efficiently as possible.
Even though quagga mussels excrete excess nutrients in Lake Havasu, most of those nutrients are absorbed in the plant material and sediments of the reservoir. As a result, CAP has not experienced a noticeable increase in nutrient concentrations in the canal itself. To ensure that nutrient concentrations are not increased as a result of non-project water introduced into the CAP system, CAP’s Board of Directors has established a Water Quality Standards Task Force to make recommendations for water quality standards.