Ice Age: The climate cooled early in the Pleistocene time and there was an Ice Age. The reason for the cooling of the climate is uncertain, but it was probably had to do with periodic variations in Earth's orbital structure and characteristics (inclination,eccentricity,precession of the spin axis). Glaciers formed on top of Steen's Mountain. The climate warmed up and the glaciers started to melt about 15,000 years ago. There was a 5,000 year span of time during which there was a large amount of runoff. During this time the runoff produced large lakes in the basins. Harney Lake and Malheur Lake are remnants of the Harney Basin Lake.
Pleistocene Lakes
| Location | Size | Depth |
| Alvord Basin | 491 Sq Miles | 200 Ft |
| Catlow Valley | 351 Sq Miles | 75 Ft |
| Harney Basin | 920 Sq Miles | 70 Ft |
CURRENT CLIMATE
Rainshadow Effects: The Coast Range and the Cascades lie between the Basin and Range and the Pacific Ocean. When clouds come off of the Pacific Ocean they are pushed up over the Coast Range. As this happens the clouds lose part of their moisture (rain). This happens again when they reach the Cascades. By the time they get to the Basin and Range area there is little moisture left. As a result there is little rain in that area. Then the clouds are pushed up over Steens Mountain where they lose most of the moisture that they have left. The Alvord Desert lies on the east side of Steens Mountain. Due to these cumulative rainshadow effects it is one of the driest places in Oregon.
Hot Days/Cool Nights: The low cloud cover in the Basin and Range area affects the temperature. It allows a large amount of solar radiation to reach the surface during the day. This produces high daytime temperatures. At night the lack of cloud cover allows the heat to escape, resulting in cool nights..
