Answer:
The directional flow of rivers is primarily influenced by the underlying geological structures and topography of a region. The reason why Sierran rivers in the Sierra Nevada mountain range mostly flow east to west, while rivers in the Coast Ranges often flow south to north, can be attributed to the tectonic and geologic history of these areas.
Sierra Nevada:
The Sierra Nevada mountain range, located in eastern California, was formed by uplift and faulting along the Sierra Nevada Fault System. The range extends roughly north-south, and the uplifted block created a steep eastern escarpment and a gentler western slope. As a result, the Sierra Nevada rivers predominantly flow from the higher elevations of the mountains in the east toward the lower-lying regions in the west, forming east-west river systems.
Coast Ranges:
The Coast Ranges, which stretch along the western edge of North America from California to Alaska, are primarily composed of multiple parallel mountain ranges and associated fault systems. These ranges formed through a complex interplay of tectonic forces, including subduction zones and the accretion of terranes. The Coast Ranges exhibit a predominantly north-south orientation, which influences the flow patterns of the rivers in this region. The rivers in the Coast Ranges often flow south to north or north to south, following the general trend of the mountain ranges.
In summary, the differing directional flow of rivers in the Sierra Nevada and Coast Ranges can be attributed to the distinct geological structures and topography of these regions. The east-west flow of Sierran rivers corresponds to the orientation of the Sierra Nevada mountain range, while the south-north or north-south flow of rivers in the Coast Ranges aligns with the parallel mountain ranges of this region.