An airborne gamma radiation detector and data acquisition system has been designed for rapid measurement of the snow cover water equivalent over large open areas. Research and field tests conducted prior to the implementation of an operational snow measurement system in the United States are reviewed. Extensive research test flights were conducted over large river basins of the north-central plains and in the high mountain valleys of the inter-mountain West. Problems encountered during development include: (1) error in the gross gamma flux produced by atmospheric radon gas daughters; (2) spatial and temporal variability in soil moisture; and (3) errors in gamma radiation count rate introduced by aircraft and cosmic background radiation. Network design of operational flight line and ground observation data used in a river forecasting system are discussed. 22 references, 4 figures, 2 tables.