Lowland river degradation of agricultural areas is a problem not only in Poland but also in Europe and worldwide. The objectives of the present study were to determine the biodiversity of the gastropod communities in lowland rivers under the impact of agriculture, the relationship between gastropod communities and environmental factors, and to estimate the usefulness of the gastropod communities as an indicator of water quality. Four rivers that flow through an agricultural area were investigated. A high value of chlorophyll a, a low value of total dissolved oxygen and a high concentration of phosphates or total phosphorus and organic matters are present in the rivers. Gastropod communities in the rivers of the Ciechanowska Upland are influenced by bottom sediments, the physical and chemical parameters of water, and the abundance of macrophytes, which are typical of eutrophic water. In the rivers of the Ciechanowska Upland, gastropods can be biological indicators of dissolved oxygen in water – Viviparus viviparus (Linnaeus, 1758) and Lymnaea peregra (O. F. Müller, 1774) can be indicators of anthropopressure. The values of diversity indices varied from site to site, but did not decrease along the river.