Lake of the Woods (Oregon) is a natural lake, and does not have a dam as portrayed in the film. (Galesville Reservoir, which is 100 miles from Lake of the Woods, was used for filming)
It seems that Josh works for a Siskiyou CSA. Siskiyou County is in California, but Josh's truck has Oregon plates.
When the threesome move away from the dam in the truck, it is very obvious from the reflections in the windshield that the car is not moving at all.
While picking up the Ammonia Sulfate it was implied that it was spring time. In all the driving scenes the landscape appeared to be Autumn.
In the opening scenes of the film, Josh and Dena are shown at Lake of the Woods, which is in southern Oregon. Later in the film, when they are heading to the lake with the boat, they mention that they will have to put in "upstream at Lake of the Woods". The following morning, Josh asks what is going on as people are reading the news, and it is said that "Someone blew up Green Peter Dam last night", and later when Josh is at the library, the news article mentions Green Peter Dam, and the Santiam River, which Green Peter Dam impounds to form Green Peter Lake. Green Peter Dam is outside of Sweet Home, and is over 100 miles from Lake of the Woods.