In the summer, a high pressure system typically builds over the entire Pacific northwest and pushes storm systems far to the north. Oregon receives very little rainfall. Because the coastal mountains build no snowpack in winter, have steep, small drainage basins, and have relatively thin soil cover, there is no groundwater reserve to sustain river levels during the summer drought. Coastal streams therefore dwindle. Summer freshwater input to the estuary is very low.
Sand spits divide and protect the estuarine environment from the dynamic influence of the ocean.
In summer, this large volume of moving sand, coupled with low estuarine outflow, allows the sand spit to move into the mouths of estuaries and perhaps, if no jetties have been built, across the channel altogether. In winter, low pressure systems move back in over the northwest coast, bringing storms which blow onshore from the south or southwest. A strong northward flowing current, the Davidson Current, moves great quantities of sand northward along the coast. These storms drop tremendous amounts rain onto the coastal mountains that discharge intoand throughestuaries. Combined with high tides and storm-generated high sea levels, the vigorous streamflow removes some of the sand spit built during the summer.