The Horse Heaven Hills are a long range of high, rolling hills in Klickitat, Yakima, and Benton counties in Washington. The hills are an anticline ridge in the Yakima Fold Belt formed by north–south compression of lava flows in the Columbia River Basalt Group. The highest point is Bickleton Ridge in the west end of the hills. They lie within the rain shadow to the east of the Cascade Range, making them significantly drier and hotter than regions west of the Cascades.
The region has been inhabited by Native Americans for centuries, who had settlements surrounding the Horse Heaven Hills. They used the range both as hunting grounds and as a geographic boundary between different tribes. The Lewis and Clark Expedition represents the first known Europeans to reach the area. European settlement followed, with the introduction of modern farming techniques. In recent years, wineries have become an important economic driver in the region and the Horse Heaven Hills AVA was established in 2005.
North-to-south compression of flood basalts that erupted several million years ago is responsible for the uplift that created the hills. They were further shaped by massive floods that occurred toward the end of the last ice age. The floods contained icebergs that brought glacial erratics, which stand out from the basalt that dominates the Columbia Basin.
Native grasses and shrubs dominate the range, which is mostly treeless. Flora also includes native flowers, some of which are listed as being threatened. There were at one time large numbers of big game roaming the hills, but many of them were hunted to extinction. Horses briefly roamed widely through the hills, but they were largely removed to facilitate farming. Today, wild horses are confined to near Satus Pass. There have been recent efforts to reintroduce animals that had previously been driven from the range.
Geography
The Horse Heaven Hills lead eastward from the Simcoe Mountains to Wallula Gap. The range is bounded in the west by Satus Creek near Bickleton, the Columbia River in the east and south, and the anticline ridge that roughly follows the Yakima River in the north. The Simcoe Mountains lead westward from the Horse Heaven Hills to the Cascade Range.
The north slope of the hills is especially steep, with the south slope being relatively gentle by comparison. This hills form the south end of the Yakima River Valley. The highest point is Bickleton Ridge which reaches high near the town of Bickleton at the southwest end of the range.
The area is sparsely populated, with only a handful of unincorporated communities such as Bickleton and Horse Heaven. Adjacent to the north side of the hills are much larger communities, such as the Tri-Cities and Prosser.
There are numerous county roads that traverse the range. It is crossed by Interstate 82 (cosigned with US 395), which connects Kennewick to Plymouth, and State Route 221, connecting Prosser to Paterson. State Route 397 travels through the foothills of the range to connect I-82 to Finley, passing to the north side of Jump Off Joe. State Route 14 crossed the Horse Heaven Hills before I-82 was built, but that designation was truncated to Plymouth when the interstate was built along the same route as the highway.
Several intermittent streams rise in the Horse Heaven Hills, including Amon Creek. The north slopes of the range drain to the Yakima River, a tributary of the Columbia. The rest of the range drains directly into the Columbia River itself.
Climate
The Horse Heaven Hills are located in one of the hottest parts of Washington, sharing in the same semi-arid climate that is experienced in nearby cities such as the Tri-Cities and Hermiston. Throughout the range, the Köppen climate classification includes warm-summer Mediterranean in the west, cold semi-arid in the east, with some smaller areas of hot-summer Mediterranean.
Rainfall values are low, ranging from in lower portions of the range to near annually in higher elevations to the west. This is because the hills lie east of the Cascades. The Cascade Mountains cause a rain shadow that makes nearly the entirety of eastern Washington and Oregon semi-arid, and much drier than areas to the west of the Cascade Crest. Most precipitation falls during the winter, with the dry season occurring during the summer. Despite the low average rainfall, summer thunderstorms can stall over the hills causing flash flooding. Flooding was most severe in Kennewick before construction of the Zintel Canyon Dam in 1992.