A neighbor just built a new driveway about 100 feet to a spot where he plans to park a 25' camper trailer. The new road has very little grade and crosses two seasonal streams.
If you build a dirt road in central Washington, you'll quickly find that the combination of rainwater and clay can stop any vehicle.
It's best not to put off installing culverts. Get it done before mid September if you can. Once it starts raining, the heavy equipment work itself is almost as damaging as having your road wash out. Your first few springs you'll have a lot of silt. Get at least 12" culvert so you can get a shovel or hoe into it to clean it out. Near Wenatchee: United Pipe, 509-662-7128, or Tumwater Drilling, 509-548-5361.
Crushed rock is a very good idea. A 4" base course of "1 1/4 inch minus" rock will give you a good, solid road surface year-round. My neighbor might want to put a load of smaller "5/8 inch minus" under the camper trailer and in the area immediately around it, where he won't drive but will want to walk and have a patio area. A load gets you about 10 feet of one-lane roadway. Bergrund Construction in Peshastin has a small dump truck that can maneuver in tight spaces. He spreads the rock evenly, so you don't need to do any smoothing or grading. Dan Dietrich, 509-669-7908.
This driveway was simple in design, but difficult to plan and to construct due to the many constraints -- side slope, varied geology, overstocked trees, property lines and rights of way. If you want to know more about road design, the WA DNR or UW Extension/Forestry have handbooks that are cheap or free.