Two related species of freshwater mollusks native to Eastern Europe, quagga mussels (Dreissena bugensis) and zebra mussels (Dreissena polymorpha), have now become damaging invasive species across the eastern and central United States, in parts of California, and in much of the Colorado River Basin (Figure 2). It is believed that both mussel species arrived in the Great Lakes in the 1980s via ships from Europe that inadvertently carried them in ballast water tanks. Both the mussel adults and larvae (veligers) can also attach themselves to boat hulls, and the transport of recreational boats from a mussel-infested water body to a previously uninfested one has driven the spread of mussels within the U.S., and to the Colorado River Basin.
Once established in a new water body, these invasive mussels attach themselves to any available surface by the thousands (Figure 1), encrusting, clogging, and otherwise impairing infrastructure for water supply and hydropower. Invasive mussels also degrade recreational opportunities in infested water bodies and rivers, including the need for time-consuming boat inspections and decontamination.
In the process of their filter-feeding, quagga and zebra mussels also filter large volumes of water–about 1 liter per day per individual. This disrupts the aquatic food web by decreasing the availability of zooplankton for native species, and increased water clarity–which, counterintuitively, can contribute to proliferation of aquatic weeds. The invasive mussels also outcompete native mussels, and harm other species by attaching to their bodies and crowding them out.
Quagga and zebra mussels have some predators in the Colorado River system, such as Redear sunfish, smallmouth bass, and crayfish. These predators offer some measure of biological population control, but also a mechanism for the toxins and microorganisms found in the mussels to move up the food chain and bioaccumulate in larger fish and birds, and potentially in humans too.
Since the mid-2000s, Federal, state, and local water agencies and natural resource agencies in the Basin have spent many millions of dollars annually on detection, monitoring, and mitigation of invasive mussels to contain their spread. The monitoring methods include analyzing water samples with microscopy to detect veligers, using gene sequencing of environmental DNA (eDNA) to confirm species presence, placing substrates (e.g., plastic disks) in waters to check for mussel attachment, and visually inspecting shorelines and infrastructure. Hundreds of inspection stations have been set up at lakes and reservoirs throughout the basin to check recreational boats entering and/or leaving the water, and at some stations, to also decontaminate vessels with scalding water.
Because of their fast proliferation and hardiness, once established in a water body, invasive mussels are very difficult to control, let alone eradicate. Aqueducts and reservoirs can be dewatered, which eventually dessicates the mussels, but this is often infeasible. Biocidal agents (e.g., chlorine, copper compounds) can be applied to the water to kill mussels directly, but this is only practical for smaller waterbodies, and can have deleterious effects on other species.
Originally found in the Ukraine, the quagga mussel varies from pea-sized to quarter-sized as an adult. They have variable coloration and can be hard to distinguish from zebra mussels. Quagga mussels were first discovered in the Colorado River Basin in 2007 in Lake Mead, Lake Mohave, and Lake Havasu. In 2012, quagga mussels were found in Lake Powell. Quagga mussels are now established throughout the Colorado River system below Lake Mead, including in the Central Arizona Project and all infrastructure in California receiving raw Colorado River water.
There is great concern about quagga mussels spreading in the Upper Basin beyond Lake Powell. Individual mussels have been found in several reservoirs in western Colorado and in northeastern and southwestern Utah, but self-sustaining populations have so far failed to establish.
Zebra mussels are native to southeastern Russia and while also variable in size, they are generally smaller than quagga mussels. They typically have distinctive dark stripes on their shells. In the Colorado River Basin, zebra mussels have thus far established only in Highline Lake near Grand Junction, in 2022. Zebra mussels were found in 2008 in Electric Lake (Utah) and in Grand Lake (Colorado), but did not establish. In lakes elsewhere in which quagga and zebra mussels do co-occur, quagga mussels tend to outcompete zebra mussels and have broader habitats, extending to deeper, colder regions of the lake.
Data and tools
The NAS database is a central repository for accurate and spatially referenced reports of nonindigenous aquatic species in the U.S. Figure 2 is based on data from the NAS.
Reclamation’s RISE database includes invasive mussel monitoring data, and reports on Reclamation projects investigating mussel detection and control methods; enter “mussel” in the search bar.
The Glen Canyon National Recreation Area maintains mussel updates on its website, including current watercraft decontamination strategies and requirements.
The Invasive Mussel Collaborative collates ongoing prevention and removal projects across the United States, including several which extend through parts of the Colorado River basin.