Beginning at the towns of Marshfield and Niangua in Webster County, Missouri and flowing north and northeast through Dallas, Laclede and Camden Counties, the Niangua River empties into the Lake of the Ozarks a few miles southwest of Camdenton. As the crow flies, the river basin is some forty-five miles long with a drainage area of 627 square miles. The river itself, however, is extremely crooked, the stream meandering to the extent that it totals 140 miles in length. Before the Lake of the Ozarks was formed by the construction of Bagnell Dam in 1931, the Niangua flowed into the Osage River just upstream from Old Linn Creek. Although in dry seasons the upper reaches of the Niangua reduces to only pools of water with little flow, further down, due to the output of a number of good sized springs, the flow becomes steady the year around. One of these is Bennett Springs located just above the Missouri Highway 64 crossing. The name “Niangua” has Native American origins, and many fanciful English translations have appeared. One authority presents convincing evidence that the proper translation is “Winding Stream of Many Springs.”

After the Louisiana Purchase, and the coming of Missouri statehood in 1821, a trickle of white settlers began to appear in the Niangua River basin. The country was rugged and wooded and yielded slowly to these early people. Small bottom farms and timber cutting comprised the main activity including tie hacking and rafting. By 1850, small water power grist mills appeared, thirty-three such mills being erected on twenty-five sites up and down the river through 1920. The possibility of the production of hydroelectric power on the Niangua River came under consideration in the early 1900s. Attention was especially attracted to one potential site where a natural cave or tunnel pierced the base of a narrow ridge, bypassing a meander of the river, thus affording the opportunity of obtaining an artificial fall of water and creating a forty-foot head for a hydraulic turbine.

The site was located in the NW quarter of Section 19, Township 37, Range 17 in Camden County, about twelve miles by road southwest of Camdenton. Although water from the natural flow of the river by-passed the tunnel at a lower elevation, most of the flow could be diverted through it by construction of a dam. The combination of the meander cut-off and building of a dam could produce a respectable forty foot head for a hydraulic turbine.

A small city was built on the site and a force of about 350 workmen lived there during the construction of the dam. A new tunnel was driven through the narrow ridge. A concrete mixer was placed on top of the hill above the dam site and concrete was sent down a chute on each side–on one to the dam and on one to the power plant. It is interesting to note that all machinery and equipment installed in the powerhouse was transported from the top of the ridge by means of an inclined track and cable car. After construction was finished, the cable car was continued in use for the convenience of the plant operators and to move materials. This arrangement lasted until 1954 when a roadway replaced the car and track.

The Niangua Hydraulic Generating Station, situated on the Niangua River, approximately 24 miles northwest of Lebanon, was completed late in 1930 by the Management and Engineering Corporation and placed in operation by the Missouri Electric Power Company on November 9, 1930.  Sho-Me Power Electric Cooperative acquired the facility in the mid-twentieth century.

Plant production is limited to a peak capacity of 2,400 kW and annual production of 7,500,000 kWh to over 12,000,000 kWh, depending on useable water flow. Operation of the plant is now integrated into the requirements of Associated Electric Cooperative of Springfield, Missouri.

Although in 1997 Tunnel Dam is small compared to the huge capabilities in both hydro and thermal generating units now available to Associated Electric Cooperative with which it is integrated, it still can help in a small way on peak periods and provide some economies in production of energy. Aside from this, Tunnel Dam can be regarded as a viable and unique sixty-seven year old engineering work. It has been so listed in a survey of historical engineering sites in the State of Missouri conducted by the Department of Social Services at the University of Missouri-Rolla.