The park's main river is the Rapa River (Rapaätno). It originates from the glaciers of Sarektjåkkå and runs down the Rapa Valley as far as Lake Laitaure, and continues outside the park, where it joins with the Lesser Lule River which becomes a tributary of the Lule River at its confluence. This river is fed by thirty glaciers, contributing to a significant flow. The specific flow, the ratio between the average flow and the drainage basin, from these waters, is the most significant in Sweden. The flow fluctuates strongly with the seasons, having an average of 100 m3⋅s−1 in July and about 4 m3⋅s−1 in winter, resulting in an average annual flow of approximately 30 m3⋅s−1. The river also transports a significant quantity of sediment. In summer, it can carry up to of sediment daily. In winter it only carries a few tons every day, resulting in an annual total of . The sediment gives the river a grey-green colour and forms large deltas. The main delta is formed at the confluence of the Rapaätno with its principal tributary, the river Sarvesjokk.