Clark Canyon Reservoir Recreation Area is a great single or multiple night spot and is easy accessible off I-15 in southern Montana.  The recreation area, run by the Bureau of Reclamation, has 9 campgrounds some of which have hookups with fees.  We stayed at the Beaverhead Campground which has about 10 or so sites and is free.

 
Campground: Beaverhead Campground
Location: Dillon, Montana (20 miles south)
Site: 7
Cost: $0
Services: None
Comments: A lovely dry camping area with large, spacious sites.  The campground is literally right off of I-15 which makes for an easy overnight spot.  During peak season it may be difficult to get a site here.

 
There are covered picnic table shelters with site numbers on them and you basically park anywhere near there, i.e., there is no site delineation.  But there are other areas with just picnic tables and fire rings and grass which look like sites. I think you can basically park anywhere as long as you aren’t interfering with others.

 

It is a beautiful area but can be closed and/or covered in snow during the winter months and probably pretty crowded during the spring/summer/fall months. We have only stayed here during late fall or early spring and it was pretty much empty both times.  While here, we had lovely weather with temperatures close to 70 degrees F during the day and then down to just above freezing at night.  We love this campground.

 
From Clark Canyon we continued on I-15 to I-90 and then headed west to Coeur d’Alene, Idaho. The roads were bare and we had very little precipitation until we got closer to the Western Montana/Northern Idaho border where the fog rolled in and the rain started. Luckily the temperature was well above freezing so the passes were fine to go over.  Hello to the snow, mountains, and pine trees and So Long to the desert.