Lewis and Clark's entry into to the American West in search of an inland Northwest Passage is considered among the greatest expeditions in American history. The Corps of Discovery were also lucky as their travels west of the 100th meridian occurred during a window of generally favorable climatic conditions. Use of reconstructed summer Palmer drought severity index (PDSI) values from 17001978 indicate that drought frequency at locations along the Lewis and Clark trail ranged from 4 to 12 yr and that the probability of encountering a drought either on the outbound or return trip approached 50 at some locations. Exact date comparisons of meteorological conditions during periods of extended encampment (i.e., 15 months) between 180406 with long-term records of nearby weather stations indicate that the Corps of Discovery avoided drought and traveled during a cooler and/or substantially wetter period than historical averages. Examination of reconstructed Southern Oscillation index (SOI) and Pacific decadal oscillation (PDO) values suggest wetter conditions prevailed in 180406 because of the co-occurrence of La Nina conditions during a cold PDO phase. Although the Corps of Discovery suffered hardships because of the wetter conditions, they avoided the more serious consequences of severe droughts that occurred in 1800 and 1808. Drought conditions along the semiarid and arid portions of the trail would have reduced forage yield for the game that were their principal source of food and increased their chances for starvation. Additionally, lower streamflow conditions along their principal navigation routes would have required greater effort and time to haul their supplies to the Continental Divide, perhaps delaying their expedition by a year.
Bulletin of the American Meteorological Society – American Meteorological Society
Published: Sep 19, 2004
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