Climatology of the interior Columbia River basin

This work describes climate means and trends in each of three major ecological zones and 13 ecological reporting units in the interior Columbia River basin. Widely differing climates help define each major zone and reporting unit， the pattern of which is controlled by three competing air masses: marine， continental， and arctic. Paleoclimatic evidence and historical weather records show that the region has undergone significant fluctuations in temperature and precipitation as air masses alternate dominance over the basin. The major change in climate occurred near the time of western settlement with the end of the “Little Ice Age.” Since then there have been numerous annual cycles in climate that may be related to the Pacific Decadal Oscillation. During the last 50 years， winter precipitation has decreased slightly and summer precipitation has increased throughout most of the basin. At the same time， winter temperatures have increased and summer temperatures have slightly decreased. Some impacts of changes in climatic means and trends on ecological conditions in the basin are discussed.