I've been up these roads, to the lake and even miles beyond where the road ends in the head of a valley.  Each time the trip is different depending on the season (how green or brown) or amount of water in the river.  In the dry season they add about 10 feet of earth behind the cement dam to help divert more water to the irrigation canals on either side of the river.  The south side of the river is Colima and to the north is Jalisco - both with lots of farming on the river delta.

There is a little town of Marabasco just south of the bridge with a No-Tell motel along the way.  Turn left just before you reach town and you can follow the irrigation canals up to the lake.  The area is popular for fishing and cooling off during family outings.  Some kids  told us about a well on the Colima side that they say dates back to the times when the indigenous peoples lived there - it's possible!