The Stillwater Bridge, connecting Stillwater, Minnesota on the west with Houlton, Wisconsin on the east, is a relatively unaltered 10-span, 2-lane highway crossing of the St. Croix River. In addition to its 10 spans, the bridge includes a counterweighted, tower-and-cable, vertical-lift span of the Waddell and Harrington type. At the site of the Stillwater Bridge, the St, Croix River is approximately 1,800 feet wide. The bridge itself, however, spans only about 1050 feet. The remaining distance is covered by a earthen causeway, which was built out from the Wisconsin shore to reduce the grade difference between the opposing banks, as well as to lower the fabrication costs of the bridge. Resting on reinforced concrete piers and abutments, the bridge superstructure displays, from east to west, the following sequence of spans: 1 concrete-slab approach span, 5 fixed steel trusses, 1 vertical-lift span, 1 fixed steel truss and 2 concrete-slab approach spans.
The six fixed truss spans are of similar size and configuration. Measuring approximately 140 feet in length, each is a 7-panel, riveted, through Parker truss with angle-iron portal, top-lateral and sway bracing. The webs are further stiffened by horizontal, angle-iron bracing across the four center panels. Except for the top chord, which consists of heavy paired channels tied with cover plate above and X-lacing below, the web members are built of paired, back-to-back angles tied with batten plates (as in the bottom chord and diagonals) or V-lacing (as in the verticals).
The vertical-lift span is also a 140-foot, 7-panel, Parker through truss. In its method of operation, the span embodies a design originally developed by J.A.L. Waddell in 1892 and subsequently refined in partnership with John Lyle Harrington. The general type is customarily known as a "Waddell and Harrington vertical lift." The span is raised and lowered by steel cables passing over sheaths at the top of steel towers mounted on the span's piers. To ensure easy movement, the span is counterweighted by concrete blocks that travel up and down within the tower framework. Originally, the motive force was supplied by a gasoline engine, which was replaced by a 25-horsepower electric motor in 1980. The control machinery is sheltered in a welded, plate-steel, gable-roofed "operator's house" mounted at roadway level on a steel framework at mid-span on the north (upstream) side. Reduction gears and winding drums for the cables are located beneath the house. With the span in raised position, vertical navigational clearance is 57 feet above normal pool elevation. The span itself is engineered for a rise of 48 feet, although an additional 3 feet of lift are available for emergency situations.
Measuring 23 feet in width, the bridge's concrete deck is bordered on the north by an angle-iron railing and on the south by a concrete sidewalk with an ornamental metal railing. The sidewalk is cantilevered on metal brackets. Although ornamental street lights have been removed from their newel posts along the sidewalk, a few of the original lighting fixtures remain on the concrete railings at the westernmost, concrete-slab, approach span. The concrete deck was rebuilt in 1973, as was the east-shore, concrete-slab, approach span in 1979. None of these alterations has significantly affected the bridge's integrity. The vertical-lift span remains in operation during the May-October navigation season.