We always are prepared with extra rain jackets and extra gloves. But you should always bring a quality rain jacket, a warm hat, hand warmers and extra gloves.
Starting at the bottom, warm wool style socks, heavy duty long johns or fleece style pants (denim jeans are a terrible insulation layer). A warm base layer for your undershirt, then a long sleeve shirt over that. A good wool layered sweater and then a puffy jacket should suffice. If another layer is needed then a quality rain jacket breaks the wind and keeps you dry.
Dressing in layers allows you to either put more layers on when the weather dictates it or shed layers when you need too. Montana’s weather can be unpredictable and being prepared will insure that you will be comfortable in any instance—making sure that you will be comfortable all day long.
There’s always room in the boat to bring any additional layers you may need. Bring more than you think you will need and you will never be unhappy.