The Cahokia Plant, now closed, and the Venice Plant used fossil fuels, a boost to UE's peak energy output.

In the late 1920s and early 1930s, UE built Bagnell Dam on the Osage River in central Missouri to harness power for a second UE hydroelectric plant. Building the dam also created the popular tourist area, the Lake of the Ozarks - one of the world's largest man-made lakes. The lake, with nearly 1,400 miles of resort area shoreline, still provides fishing, boating and recreation for all ages.

Electric power generated at the Osage Plant was not destined to satisfy Union Electric customers' demand for energy for long. At the outset of World War II, UE retired the first unit at Venice Plant, and construction began on two new units there. Each new unit featured larger and more efficient generators which produced power at lower cost. But demand for electricity had not grown quite as fast as power-producing capabilities, leaving part of the UE system uneconomically idle.

To address this dilemma, in the late 1940s, UE joined a system of interconnected power companies bringing a new dimension to its operations. By constructing connected transmission lines, Union Electric and other utilities created a "power pool" to share electricity.

Building Bagnell Dam, completed in 1931, created a popular mid-Missouri tourist area, the Lake of the Ozarks.

The "power pool" arrangement worked well, and UE is now linked to 28 other power providers.