The Flat Brook, also known as the Big Flat Brook, is a scenic trout stream in northwestern New Jersey. It originates below the dam at Saw Mill Lake in High Point State Park and flows southwest for approximately 20 miles before joining the Delaware River at Walpack Bend near the hamlet of Flatbrookville. The river is fed by numerous springs and small tributaries as it winds through the forested hills of Sussex County.

Paddling opportunities on the Flat Brook are limited due to its relatively small size, but it offers excellent fly fishing, particularly for trout. The river is primarily Class I, with a mix of gentle riffles and calm pools. While specific rapids are not notable, the stream provides a peaceful float suitable for novice paddlers and anglers alike. The best paddling is typically in spring and fall when water levels are higher.

The Flat Brook and surrounding area support diverse wildlife. The river is home to various fish species including rainbow trout, brown trout, minnows, sculpin, and dace. The riparian habitat hosts aquatic insects like stoneflies and caddisflies. Paddlers may spot wading birds, waterfowl, and potentially beavers along the banks.

Key landmarks include the Saw Mill Road put-in and the confluence with the Delaware River. The river flows through the Flat Brook Wildlife Management Area, offering paddlers a chance to observe nature in a protected setting. Access points are limited, with the main put-in near High Point State Park and take-out at Flatbrookville.

Historically, the Flat Brook valley was home to small farming communities and mill operations. Today, it represents one of New Jersey's most pristine trout streams and is valued for its natural beauty and recreational opportunities.

Safety considerations include being aware of water levels, which can fluctuate seasonally, and watching for submerged obstacles. Anglers should note that special regulations apply to certain stretches of the river to protect the trout population.

While multi-day trips are not typical on the Flat Brook itself, paddlers can combine a day on this river with extended trips on the nearby Delaware River for a more comprehensive regional paddling experience. The Flat Brook offers a serene paddling and fishing experience in a picturesque setting, making it a hidden gem among New Jersey's waterways for those seeking a tranquil outdoor adventure.