Based on the latest information available, the boat ramp named Old US Highway 91 in Cascade County, Montana is a concrete boat ramp with a single lane. It is approximately 12 feet wide and 56 feet long, providing ample space for launching and retrieving boats.

This boat ramp services the Missouri River, which is a major waterway in Montana. It is a popular destination for recreational boating, fishing, and other water activities. The Missouri River also provides an important source of water for irrigation and other uses.

According to the Montana Fish, Wildlife & Parks website, all types of watercraft are allowed on the Missouri River, including motorboats, sailboats, kayaks, canoes, and stand-up paddleboards. However, boaters should be aware of the river's changing water levels, strong currents, and potential hazards such as rocks and logs.

Overall, the Old US Highway 91 boat ramp is a convenient and well-maintained access point for boaters looking to enjoy the Missouri River in Cascade County, Montana.