This study was conducted on a storage reservoir situated in a valley, in the lower course of the Sząbruk stream in north-eastern Poland, the Olsztyn Lakeland mesoregion. The catchment area of the Sząbruk stream consists of an agricultural and an afforested part. A storage reservoir is found in the lower part of the Sząbruk stream valley. The reservoir was built 25 years ago. It is enclosed by a dike and equipped with an outlet box. Outflows from the reservoir pass through the terminal segment of the Sząbruk stream to Lake Wulpińskie. The results of the experiment indicate that the calcium content of water evacuated from the catchment was determined by the type and intensity of catchment use, ranging from 22.3 mg Ca dm-3 to 178 mg Ca dm-3. The highest calcium concentrations, 113 mg Ca dm-3 on average, were noted in the agricultural catchment connected to a drainage network; lower levels, 78.7 mg Ca dm-3, were found in farming areas drained via ditches, while the lowest Ca content in water, 38.7 mg Ca dm-3 on average, was determined in outflows from afforested catchments. Calcium concentrations were lower during the growing season in all studied catchment types. The calcium load discharged from the catchment dependnded on the catchment management. The highest calcium loss per area unit was observed in the drained agricultural catchment (76.6 kg Ca ha-1 year-1), followed by the catchment drained via ditches (56.3 Ca ha-1 year-1) and the afforested catchment (31.8 Ca ha-1 year-1). Despite the inflow of calcium-rich drainage water, calcium concentrations decreased by 11%, from 56.8 mg Ca dm-3 to 50.3 mg Ca dm-3, after the stream’s waters passed through the storage reservoir. An increase in Ca levels was noted in the girdling ditch. The flow of water through the ditch minimizes sedimentation, and higher quantities of Ca were supplied with drainage water. The reservoir accumulated 242.4 kg Ca per ha in the course of one year, mostly in the growing season (83%). The above indicates high involvement of biological processes. The reservoir fulfilled the role of a barrier inhibiting calcium loss from the catchment.