The patterns that occur where the river meets the ocean create their own weather. One reason Astoria, Oregon, never stays hot for very long is that the heat draws the marine layer from the ocean (left in this photo) inland and chills the air.
As I understand it, the marine layer is not actually the visible cloud, but the cloud is often held underneath the marine layer, which is an invisible pressure zone. Later in the afternoon of the Regatta Parade with its 90 degree temperature, I opened my door near the river and felt positively cold. As I drove past town and stopped at the riverfront park, I discovered a finger of fog reaching upriver, a clear indication that change is coming rather quickly.
Crossing the Young's Bay Bridge to Warrenton a few minutes later, I could see the fog bank growing taller as it rolled in from the ocean. I wish I'd been able to take the next photo as the bank grew and darkened, but I thought I'd better keep both hands on the wheel, as traffic had begun to move. It was a mixed blessing. Who likes to sit in traffic, but then again, it was nice watching the atmosphere change.