Cenozoic drainage history of southern British Columbia

Anomalous geomorphology, valley-fill geology and cross-cutting relationships throughout the southern Interior Plateau of British Columbia indicate the area retains relict or exhumed landforms of Late Cretaceous and Cenozoic age. Preglacial valleys and other landforms are identified on high-resolution shaded relief maps, and their ages are bracketed by geological and structural relationships. Drainage anomalies, such as valley-floor divides, underfit streams and graded valley segments, indicate former base levels in the landscape. Reconstructions of past landscapes and waterways are presented for six time intervals spanning the last 100 million years.; Regional base level was relatively stable at 1000--1300 m elevation during the Mesozoic. From Late Cretaceous to the middle Eocene, base level fell 500--700 m, resulting in incision of valleys. Some valleys filled with sediments to an elevation of about 900 m from middle Eocene to Miocene time. Following the Miocene, base level dropped significantly, eroding canyons and exhuming older, partially buried valleys. Changes in base level over the last 100 Ma are greatest in the southern Interior Plateau and are least in the Quesnel-Horsefly area.; Regional drainage was broadly northward throughout the Cenozoic. An anabranching river system flowed northwestward across Cariboo Plateau to Fraser River valley from Quesnel and Shuswap Highlands during Late Cretaceous to Cenozoic time. Other valleys dating back to middle Eocene time and possibly earlier are those of North Thompson River south of Clearwater, Thompson River from Kamloops to Spences Bridge, Nicola Lake, Hat Creek, Bridge River, Seton River, and Okanagan Valley north of Penticton. Miocene rivers in the Coast Mountains flowed northeastward across Thompson Plateau and northward along Clearwater and Quesnel river valleys. Modern southward drainage along Okanagan, Kettle, Thompson and Fraser rivers was established after Miocene time.