This report presents data which are, in general, supplementary to those of the surface-water investigations made in the past by the Geological Survey. Those investigations have consisted essentially of the operation of the many gaging stations on the Flathead River and tributaries. The data presented were obtained from a detailed field investigation of the various manmade devices that are factors influencing the quantity or regimen of the flow at the gaging stations. These factors include diversions from the stream, bypass channels carrying water around the gaging stations, return flow from irrigation or other projects, storage and release of flood waters, and other similar factors. Where feasible, the location, size, effect upon the streamflow, periods of use, method of operation, and similar information are given. The information is segregated into sections corresponding to areas determined by the location of gaging stations. An index of streamflow records is included. A section dealing with the adequacy of .available water-resources data, including location and period of record, also is included. This information is given in general terms only, and is portrayed mainly by maps and graphs.