To determine the characteristics of waves generated by recreational craft within the Upper Mississippi River System (UMRS), 246 controlled runs were made at an Illinois River site and a Mississippi River site. Data showed that recreational boats can generate from 4 to 40 waves per event, with a mean of about 10 to 20 waves. Average wave heights varied from 0.01 to 0.25 meter, with a median of about 0.06 to 0.12 meter. The maximum wave height was as much as 0.6 meter. The data were used to develop a regression equation for estimating maximum wave heights on the basis of the speed, draft, and length of the oats, and their distance from the measuring point. Data from uncontrolled boating events on the Mississippi River indicated that as many as 704 boats passed highly used area of the UMRS in a single day on a busy weekend. Sustained movement of recreational boats can generate essentially continuous waves. During the day of heaviest boating activity, the maximum wave height measured was 0.52 meter, and the average for the day was 0.065 meter. Calculations showed that for waves of 0.4 meter in height to develop at the Mississippi River site from wind alone, the wind would have to be blowing at a speed of about 26 meters per second (58 mph) across the measuring point. Additional analyses showed that the shorelines are subjected to wave activity of fairly high intensity.