Last year we had 14 partners collecting water samples at 143 swimming hole sites across NH, VT, MA & CT. A total of 1,030 individual samples were tested for E. coli bacteria and results posted on That’s a lot of data!

So what does this info tell us? How clean or dirty are our rivers? Well, it depends on the weather. Heavy rains wash pollution into our rivers from lawns, roads, fields, and cause combined sewer overflows (CSOs) to discharge untreated sewage directly into our rivers. Bacteria levels are more likely to be at unsafe levels for 24 – 48 hours after heavy rains.

Samples collected during dry weather are clean to swim and boat 78% of the time. However, during rainy weather, sites are clean to swim and boat only 54% of the time. These are summary results, so be sure to check the data at your favorite swimming and boat location for specific information. Some sites are always clean no matter the weather!

Does your local town, agency, or watershed group test for E. coli in the Connecticut River basin? Want to join us to share your results online? Contact us to learn how you can join the ‘Is It Clean’ network.