Culvert markings

I’ve noticed that many culverts in Colorado have signs with a code. For example, I routinely drive over a place where the Blue River passes under Highway 9 in a 3-4 foot pipe. This is marked with a sign that says “Blue River, G-12-H”. I’ve noticed similar signs on many culverts but they have different letter/number codes. I recently noticed a “G-14-L”. I’m curious what the letter/number codes mean. They do not seem to be unique enough to be a serial number for the sign (and why would that be posted so prominently). I suspect they have something to do with either the location on the river or the pipes installed under the road, or both. Can any road crew workers or civil engineering types enlighten me?