Congress authorized the U.S. Army Corps of Engineers to conduct the Great Lakes Mississippi River Interbasin Study (GLMRIS) to prevent the spread of aquatic nuisance species (ANS) from one basin to the other. Here are a few fast facts about the risk assessment:

• 254 ANS organisms identified
• 39 identified as organisms of potential concern
• 14 ANS are Medium or High risk in Focus Area 1 (CAWS)
• 5 pathways in Chicago Area Waterway System (CAWS)
• 18 pathways in Focus Area 2 (non-CAWS).

The scope and magnitude of the problem produced a daunting assessment challenge, especially when scientific data on these species was often limited. Beginning with the simple risk model:

Risk = Probability x Consequence

we decomposed the sequence of events necessary for the establishment of an ANS as well as the categories of consequences that could occur with establishment. This necessary chain of events is summarized below:

Probability of spread to new waterways = P(path) x P(arrival) x P(passage) x P(colonizes) X P(spreads)