The Columbia Mountains, comprised the Purcells, Selkirks, Monashees and Cariboos, were uplifted between one and two million years ago, and include impressive peaks more than 3,000 metres high.

West of the Columbia Mountains is the Interior Plateau, which covers much of Interior B.C. and is home to the Okanagan Valley. A plateau, like a mountain, is an uplifted section of Earth's crust, which has not been subject to folding or carving. As with all uplifted areas, however, these relatively flat expanses are subject to erosion. For example, the region's rolling terrain has been moulded by river action.

A small portion of the Cascade Mountains extend into Canada just west of the Okanagan Valley. The rest stretches down to California. Making up the B.C. Cascades, from east to west are the Okanagan, Hozameen and Skagit ranges.

The Okanagan Valley was formed by glacier activity during the Tertiary and Quaternary periods. Mile-thick ice layers retreated 10,000 years ago, scraping the surface and leaving behind valuable sedimentary deposits along its borders. Additional mountain river erosion and flooding left behind nutrient and mineral rich soils which have accumulated into deltas.

The system of oblong lakes in the Okanagan Valley is a remnant of the glacial retreat. Okanagan Lake, the largest , is 111 kilometres long and ranges from about 3.2 to 6.4 kilometres wide. East of Okanagan Lake are Swan, Kalamalka (a Salish word meaning lake of many colours) and Wood lakes. To the south are Skaha, Vaseux and Osoyoos lakes. The system drains south into the Okanagan River, crosses the border into Washington, and joins the Columbia River. The Columbia then veers west and carves a path through the Cascade and Coast Mountain ranges to drain into the Pacific Ocean.