Mattawa is a desert riding area in Washington State that is popular in the Winter months with very little to no snow (usually).  The area has lots of trails and small canyons you can go up and down in.  There are no designated trails, but there are plenty of well used and obvious trails to ride.  You are not required to to stay on the trails.  The terrain consists of areas with lots of woops, rocks, sandy areas, sage brush, and dirt sections.

Directions From Mattawa:
Go through Mattawa, take a left on Road "R", stay to the left when it gets to the first cattle guard.  Once you cross the second cattle guard you are there.  You will see the little BLM kiosk on the left side of the road.