The Flat River in the Northwest Territories flows through remote wilderness terrain before joining the Nahanni River, and is best understood as two distinct stretches with markedly different characters. The upper stretch, from Paddler's Lake to Irvine Creek, is a fun and challenging whitewater river that also presents logjam hazards. The lower stretch, downstream of Irvine Creek to the confluence with the Nahanni, is the flatter, more open water that gives the river its name.

A recommended put-in is at Paddler's Lake, which offers a gentler introduction to the river. From there the water is initially calm, gradually becoming a narrow, twisting channel that remains manageable. As the river passes through the former Cantung Mine and Tungsten townsite area, the first notable rapid appears — a drop of roughly one metre over a short distance — followed by several more rapids requiring scouting. Paddlers who begin at Seaplane Lake miss this upper whitewater section entirely.

Between Gatehouse Creek and Irvine Creek, the river can be paddled in approximately two days, with a campsite at Bennett Creek along the way. This section includes a mandatory 850-metre portage trail around the Cascade of the Thirteen Steps. From Irvine Creek to the confluence with the Nahanni covers roughly 85 km and takes about two days at an easy pace. A notable feature along this lower stretch is a series of hoodoos — limestone pillars formed by differential erosion — located on the steep mountainside on river left, just upstream of the Caribou River. Parks Canada notes that some of these pillars reach 15 metres in height.

Many paddlers continue beyond the Flat River onto the Nahanni and then the Liard River, taking out at Lindberg Landing, adding approximately seven more days of travel through canyon scenery. The Flat River corridor passes through a mountainous landscape, and the transition from the mountains to lower terrain is a notable geographic shift experienced on the downstream journey.