Samples from rain collectors were collected when enough daily precipitation accumulated in the collectors. Sampled precipitation events during the study period were in May, June, July, August, and September 2018. Physical properties of sampled water-temperature, pH, and specific conductance-were measured in the field. Water samples were analyzed for stable isotopes of oxygen and hydrogen and for chloride concentration. Recharge rates for aquifers supplying springs were determined using precipitation volume and chloride concentrations for a 12-day period before the sample-collection date. Multivariate statistical analysis methods used on water-chemistry data included principal component analysis, cluster analysis, and end-member mixing analysis. Water composition was used to determine the spring type and contributing aquifers for 11 springs in the North and South Units of Theodore Roosevelt National Park from analyses of water-chemistry data between May and September 2018. In the North Unit, Achenbach Spring was classified as a filtration spring with water from an unconfined part of the upper Fort Union aquifer and infiltration of precipitation. Hagen Spring, Mandal Spring, and Stevens Spring were classified as contact springs supplied by semiconfined parts of the upper Fort Union aquifer. Overlook Spring at one time may have been a natural spring or seep but now is a developed spring that behaves like a flowing artesian well completed in a confined part of the upper Fort Union aquifer. In the South Unit, six springs were classified into two spring types: filtration and contact springs. Boicourt Spring and Sheep Butte Spring were classified as filtration springs that have water supplied by unconfined parts of the upper Fort Union aquifer and infiltrated precipitation. Big Plateau Spring, Lone Tree Spring, Sheep Pasture Spring, and Southeast Corner Spring were classified as contact springs that receive waters from a semiconfined part of the upper Fort Union aquifer. from the ground and flows or forms pools (Bryan, 1919). Springs commonly form when the side of a hill, a valley bottom, or other excavations intersect flowing groundwater at or below the local water table (U.S. Geological Survey, 2019). Seeps are a type of spring in which the water flows from pores in the ground over considerable areas (Bryan, 1919). Seeps occur from a slow movement of water through Unconfined aquifer water table Unconfined aquifer water table Rotational slump or landslide Low permeability unit Low permeability unit EXPLANATION Spring Groundwater level Groundwater flow direction Filtration Confined aquifer hydraulic head Confined aquifer hydraulic head spring or seep Flowing artesian spring Upper contact spring Lower contact spring Figure 3. Types of springs in the study area.