Marine high-productivity zones along the continents are of great economic importance, and they account for most organic carbon burial. The carbon cycle in many coastal zones is influenced by the sediments and dissolved nutrients introduced by rivers. However, there is little direct evidence for a regional marine response to changes in river dynamics. Here we present a suite of high-resolution records of organic and inorganic paleoceanographic proxies, which together demonstrate that past changes in Indus River discharge have strongly affected productivity patterns in the coastal northeastern Arabian Sea. Anthropogenic activity, including the building of dams and irrigation facilities during the past century, has drastically decreased the discharge rate of the Indus River. Between A.D. 1890 and 1998, the period over which this reduction occurred, primary productivity off the Pakistan coast seems to have decreased by more than one-third. Over the same period, the regional oxygen minimum zone weakened, increasing the supply of oxygen to the sediments, leaving the expected imprint on a suite of redox-sensitive elements and reducing the preservation of organic matter.