As noted by the USDA's Natural Resources Conservation Service (Ron Abramovich),

• the end-of-July level of the Blackfoot Reservoir, in eastern Idaho, was the lowest since 1934
• the three-year moving average for the April 1 snow water equivalent and annual (October-September) streamflow for several basins along the eastern Idaho border are at or near record low levels, reflecting the long term drought
  • Snake River at Heise reflects the Upper Snake snow/streamflow from western Wyoming above Palisades Reservoir
  • Little Lost Basin is northeast of Idaho Falls and represents basins near the Montana border
  • Salmon River near White Bird is in western Idaho and reflects the basin as a whole above Riggings

The bottom graph to the right shows the individual summer reconstructed PDSI values for each year (gray curve) and the longer-term variations (red 20-year low pass filtered curve) for 1306-1990. The annual values (gray curve) indicate that summer droughts have been as severe or worse during the last 700 years as the most severe 20th century drought (1931), specifically 1492, 1657, 1677, 1717, 1721, 1756, 1783, 1800, 1863, and 1889. A prolonged period of drought occurred during the mid-1600s, but the 1930s stands out (red curve) as the most severe drought episode in terms of combined severity and duration.

Reference:

Cook, E.R., D.M. Meko, D.W. Stahle, and M.K. Cleaveland, 1999. "Drought reconstructions for the continental United States." Journal of Climate, 12:1145-1162.