Coquille River - Tidal Effect

Tidal Effect

The Coquille River is a tidal-effect river. During high tides at sea, the water at the mouth of the river acts like a dam, impeding the flow of the stream. Actually, more water always flows out into the ocean than enters with the tide. However, logs or sticks or leaves on the surface actually float up the river at certain times of day. There is a time lag between the high tide at sea and the effect pushing upstream. This varies according to the number of miles from the ocean. The tidal-effect is observed as far as Myrtle Point, some 28 to 30 miles upriver. The head of tide is 41 miles upriver from the mouth at Bandon. But some salt water does enter across the bar. The salt water intrusion was once minor. Today, however, sometimes traces of salinity appear only a few miles below the town of Coquille. This phenomenon affects industry along the river, as mills and other sites need fresh water intakes on the river in order not to rust or damage equipment or affect production processes.

On its main forks, the Coquille is also an alluvial river valley, meaning it was once a drowned river valley—a lake, or perhaps a much wider river, with a sandy bottom. The soil in the valley is very fertile. However, the rise and fall of the water level, and flooding in winter, prevent ordinary types of farming. A lush grass grows in the valley lowlands which can survive either drought or inundation. Tide-gates along the river can bring in water when needed. On this sweeping stand of grass, in summer many cattle find pasture.