Recent work in the Canadian Cordillera has led to the formulation of several models for its tectonic evolution. These models can be tested by relating the structural evolution of the crystalline core zones and fold belts to the depositional record of the orogenic foredeeps and successor basins. They require a comprehensive understanding of paleodrainage. The arcuate geometry of orogenic belts places the principal structural constraint on the evolution of late orogenic drainage patterns. Like other mountain belts, the two orogens of the Canadian Cordillera (Pacific orogen in the west, Columbian orogen in the east) consist of a chain of structural salients and reentrants. During the growth of folds and thrust faults, the structural reentrants constituted the shortest connec ion between the rising core zones and the subsiding foredeeps and successor basins. Streams issuing through the reentrants, therefore, were the first to capture the longitudinal drainage which evolved during the growth of the fold belts and became the most important suppliers of clastics to the late-orogenic basins. This "reentrant principle" can be illustrated on a regional tectonic scale with the Peace, Liard, Eagle Plain, and Chukachida reentrants of the Columbian orogen. Local examples are used from near Crowsnest Pass, the Spatsizi Plateau, and the Fraser River. From the structural salients, predominantly small, though locally vigorous, streams issued directly into the late orogenic basins. Therefore, simple straight drainage lines connecting source areas and clastic basins were pro ably valid only for the earliest stages of uplift in the Canadian Cordillera. Later growth of folds and thrust faults produced curved or even U-shaped river systems, which merged near reentrants and effected thorough mixing of compositionally diverse sediment loads.

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