Operation of the Garrison Diversion Unit in North Dakota may have various effects on the quantity and quality of streamflow in the Sheyenne River and the Red River of the North. To model the effects that the Garrison Diversion Unit could have on water quality, gaged and estimated historic streamflow data and estimated unregulated streamflow data were compiled to develop a complete monthly streamflow record for January 1931 through September 1999 (the data-development period) for 35 sites in the Red River of the North Basin in North Dakota, Minnesota, and South Dakota.

During the entire data-development period, gaged streamflow data were available for only 4 of the 35 sites, incomplete data of various length were available for 10 sites, and no data were available for 21 sites. Drainage- area ratio and Maintenance of Variance Extension Type 1 methods were used to estimate the historic streamflow for months when no data were available.

Unregulated streamflow for the 35 sites was estimated by eliminating the hydrologic effects of Orwell Reservoir, Lake Traverse, Mud Lake, Lake Ashtabula, and surface-water withdrawals. Modeled flows at the Red River of the North at Wahpeton by the U.S. Army Corps of Engineers were used to eliminate the effects of Orwell Reservoir, Lake Traverse, and Mud Lake, and water-balance procedures were used to eliminate the effects of Lake Ashtabula.