Middle River rises in south-central Guthrie County, and its navigable section begins in east-central Adair County near Middle River Forest County Park. Flowing east through croplands, pasture, rolling forested hills, and limestone bluffs, it forms a scenic but moderately challenging paddle, especially at high or low water, with occasional chutes and ledges and the best paddling in spring, early summer, and fall. Along this stretch are man-made features including Madison County's Roseman Covered Bridge and Holliwell Covered Bridge, the historic ford at Pammel Park, and the Harmon Tunnel, originally constructed in 1858. The river eventually joins the Des Moines River near Carlisle in northern Warren County, and the designated water trail runs from Middle River Forest County Park to the historic Holliwell Covered Bridge, a route of just over 45 miles within a navigable section about 100 miles long. The Holliwell Bridge, one of the first covered bridges built in Madison County, crossed Middle River on the Winterset Indianola Road, the main highway used by pioneers heading west through Winterset. Settlers were drawn by rich soil known as the Hoosier Prairie, abundant streams, forests of sugar maple, black walnut, and towering cottonwoods, and plentiful buffalo, elk, deer, lumber, and stone. In 1880, Holliwell Bridge replaced an open wood frame bridge first built in 1855, and it became notable as the longest of the covered bridges and for its covered wood bow truss design. Madison County also escaped the most recent glaciations about 12,000 to 15,000 years ago, making it part of the Southern Iowa Drift Plain, where rolling hills are shaped by thick layers of limestone and shale formed millions of years ago when much of the Midwest lay beneath the Carboniferous Sea; along Middle River, these formations appear in striking limestone walls and cliffs, including prominent bluffs more than 140 feet high about 1 mile above Pammel Park.