The spread of invasive non-native mussels is a major concern for all of us that value rivers.  Increasingly states and managers are setting up fees, wash stations, and regulations to prevent recreationists from spreading mussels. While the most common threat is mussels "hitch-hiking" on the hulls and in bilge water of trailered boats, it is at least possible that they could be transported by whitewater craft that move from river to river. The adult mussels can attach to the bottom of boats or in mud attached to the boat. More problematic to whitewater boating is larval mussels (called veligers), tiny organisms that drift down rivers and can live in splash water inside the raft/kayak. The most sure-fire way to avoid distributing them is to make sure rafts and kayaks are clean (mud free) and dry from river to river. Adults die within about a week when out of the water and veligers can be eliminated by simply draining the boat of residual water. 

There is no comprehensive list of rivers that contain the invasive mussels, so as a rule of thumb you should take extra precautions when travelling from one watershed to another. Many rivers do in fact have the invasive Mussels including the Lower Colorado, Idaho's Bear River, and several Southern California rivers.   

The zebra and quagga mussels have explosive reproductive rates and can quickly carpet the bottom of lakes and slow-moving rivers, disrupt the food chain, and clog water works. Help keep our rivers healthy:

CLEAN AND DRY YOUR BOATS AND GEAR BETWEEN RIVERS

and especially between watersheds

More information on this issue and specific cleaning recommendations can be found here: http://www.protectyourwaters.net/