Its drainage basin, covering 4784 square kilometres, is the site of almost one-quarter of Quebec's agriculture. However, the agricultural activities of the Yamaska basin have changed a great deal. Hogs and corn are now the two most important commodities in the basin. Water quality continues to be poor throughout the watershed, despite improvements since the late 1970s resulting from the clean-up efforts by the urban, industrial, and agricultural sectors. Observations in the Yamaska basin suggest a close relationship between agricultural pressures, which have reached an exceptional level for Quebec, and the quality of the water.
Agricultural activities are the principal sources of nutrients in the basin.
About 17 different pesticides and their degradation products are detected each year in this river. On occasion, water quality guidelines are exceeded for metolachlor, cyanazine, metribuzin, MCPA, MCPB, and dimethenamide. Even when they are detected in low concentrations, the presence of several pesticides raises concerns because of the possibility of still-unknown additive or synergistic effects on aquatic life. Further improvement in the quality of Yamaska waters depends on continued promotion and adoption of good agricultural practices, particularly that of reducing the use of mineral fertilizers. In one case the concentration of atrazine in water at a single subsurface drain outlet in a small stream was 14 micrograms per litre, well above the drinking water guideline.