Yes, it is a tailwater, and much cooler than the LBH. But "cold
water" must be used carefully, especially in relation to other Montana
cold water streams.

From MT FWP:

"Prior to 1965, the Bighorn was a warm, silty stream that flowed out
of the Bighorn Canyon into the Montana prairie. With the completion of
Yellowtail Dam, it transformed into a cooler tailwater ideal for trout.
It can be called a coldwater fishery, though not in the way some of
colder, crisper streams in the central and western parts of the state
can. A species list shows the middle-ground diversity of the Bighorn
River:

Bigmouth buffalo, black bullhead, burbot, channel cat, carp, emerald
shiner, flathead minnow, flathead chub, freshwater drum, goldeye, green
sunfish, longnose sucker, whitefish, river carpsucker, sauger, shorthead
redhorse, smallmouth bass, smallmouth buffalo, walleye, white sucker,
yellow perch.

The river meets the Yellowstone near Custer, the Yellowstone turning
into a warmwater fishery east of Billings"


Our definitions of warmwater are just different due to our geographic
location. If you are from the Heartland or other areas of the midwest,
I'm sure the Bighorn seems like coldwater heaven. But when you live in
a state that has some of the highest, cleanest, coldest, and most pure
water left on the planet....the Bighorn seems, well, pretty warmwater.

Anywhere I can catch a trout, a ling (burbot), and a catfish in the
same day......warm water.

Not trying to stir things up. I just figure that if you come to MT,
which is known for it's mountains, fishing, and some of the purest
waters left on earth, why fish a river that probably looks like a river
near you. The Bighorn is a man-made fishery...and it looks and feels
like one.

You will catch big fish all day long, yes. But you are comparing
their fight to a fish caught in a river near you, not a river near me.
A wild 12 inch trout caught in the cold waters of 10,000ft will fight
harder than a man-planted 22 incher in the warmer waters of the Bighorn.

If you ever get the chance to eat a trout caught at 10,000 ft and
compare it to a Bighorn trout, or a trout near you...you will know what
I mean. There is an actual physical difference in the flesh due to
water temp and comparative inactivity of fish on the man-made fishery
that is the Bighorn.