To expand agricultural production and address water scarcity, India is moving forward with the National River Linking Project (NRLP), which will connect 44 rivers via 9,600 km of canals. Here, we compile the first complete database of proposed NRLP dams, reservoirs and canals, including operating schedules for Himalayan infrastructure. We evaluate potential NRLP-derived changes to mean annual water discharge for 29 rivers and mean monthly water and sediment discharge for six rivers flowing to five major deltas. Sediment rating curves are used to quantify the impacts of changing water discharge within the rivers, and basin-wide trapping efficiency is established for new reservoirs. Given full implementation of the NRLP, we forecast reductions in annual suspended sediment transport to deltas of 40–85% (Mahanadi), 71–99% (Godavari) and 60–97% (Krishna) due to profound reservoir trapping and peak streamflow reductions. The Ganga before its confluence with the Brahmaputra is projected to experience a 39–75% reduction in annual suspended load. The Brahmaputra before its confluence with the Ganga is projected to experience a 9–25% reduction in suspended load, despite losing only 6% of its annual water flow. We calculate a projected corresponding aggradation decrease for the Ganga-Brahmaputra delta from 3.6 to 2.5 mm y –1 , which is a large enough change to drive relative sea-level rise at the delta front. At the remaining four deltas, the NRLP will exacerbate current sediment starvation. We reconstruct the annual water transfer volume proposed for the NRLP to be 245 km 3 y –1 , higher than previous estimates due to the inclusion of along-canal usage. If completed, the NRLP will transform watershed boundaries, with more than half of the land in India contributing a portion of its runoff to a new mouth. These impacts may have profound environmental and public health implications, particularly in the context of future climate change.

We present an improved water-scarcity metric we call water depletion, calculated as the fraction of renewable water consumptively used for human activities. We employ new data from the WaterGAP3 integrated global water resources model to illustrate water depletion for 15,091 watersheds worldwide, constituting 90% of total land area. Our analysis illustrates that moderate water depletion at an annual time scale is better characterized as high depletion at a monthly time scale and we are thus able to integrate seasonal and dry-year depletion into the water depletion metric, providing a more accurate depiction of water shortage that could affect irrigated agriculture, urban water supply, and freshwater ecosystems. Applying the metric, we find that the 2% of watersheds that are more than 75% depleted on an average annual basis are home to 15% of global irrigated area and 4% of large cities. An additional 30% of watersheds are depleted by more than 75% seasonally or in dry years. In total, 71% of world irrigated area and 47% of large cities are characterized as experiencing at least periodic water shortage.