Radium isotopes (223Ra, 224Ra, 226Ra and 228Ra) have been measured in Cayuga Lake, one of eleven Finger Lakes in central New York State, and in several of its tributaries during and subsequent to the 2001 spring thaw. The long-lived isotopes (228Ra, t1/2=5.8y; 226Ra, t1/2=1601y) were useful in identifying and monitoring the movement of a water mass that was introduced into the lake through a ground-water source as opposed to surface input from tributaries. Differences in the 228Ra/226Ra isotopic ratio of the ground water, in contrast to lake and tributary water, enabled the water mass to be traced for one month after introduction into the lake. The short-lived isotopes (223Ra, t1/2=11.1 days; 224Ra, t1/2=3.6 days) were used to estimate the time of emplacement of the water mass and calculate current velocities by observing the 223Ra/226Ra and 224Ra/228Ra activity ratios of the water mass through time. The short-lived isotopes were also used to identify areas of input of recently added spring-thaw water by their elevated activities compared to pre-thaw lake water, where activity was at near-zero levels. At Myers Point Delta, where Salmon Creek enters the lake, the short-lived isotopes allowed differentiation between ground water and surface water input into the lake. Several kilometers farther south, an area of active inflow was detected throughout the monitoring period, and mixing relationships were clearly evident. This study demonstrates the usefulness of radium isotopes in understanding lake dynamics in areas such as: 1) water mass tracing, 2) ground-water, surface-water and lake-water sources, and 3) mixing behavior. Application of this technique to lakes should be of value to limnologists and resource managers in better understanding long term behavior of ground-water dynamics in lakes, a particularly difficult source to characterize.