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The Spanish River is located in northern Ontario flowing into Lake Huron at the western end of the North Channel. Its drainage basin is approximately 6500 square kilometers above Agnew Lake. The flow of the river is tapped four times for the production of electricity. Three sites currently owned by INCO Ltd. are Big Eddy, High Falls and Nairn Falls. The fourth site at Espanola is owned by the E.B. Eddy Company. The major tributary of the Spanish is the Vermillion River. It joins the Spanish about ten kilometers up stream from Espanola and drains a large area to the east of the main watershed.

Spanish R. map

Agnew Lake itself is the result of the building of Big Eddy dam. Completed in 1920, the dam is made of concrete, spanning 361 meters with a maximum height of 45 meters. The standard operating "head" for the turbines of the Big Eddy power plant is 31 meters with a full lake. At "full lake", with all spillways, "Johnson Valves" and turbines at full power about 1000 cubic meters of water per second can be sent down the river. This is about 15% of the water that goes over Niagara Falls each second. At this flow rate, major flooding is caused downstream. Fortunately this seldom happens, with the last occurence being in May 1960.
The Spanish River valley was a major spillway for meltwater from the retreating glaciers. Many parts of the valley show classic geomorphic features of spillways including large sand flats, levees, potholes in the rock (at the High Falls itself), layered gravel deposits, varved clay deposits and high benches. There is every indication of periodic ponding followed by high energy erosion.
Before the dams were built, the river flowed down from the site of Big Eddy dam and split into two streams, one turning south and east following the course over the present day falls. The other stream continued east over another falls and passed through where #2 generating station is now. The site was refered to as Twin Falls by the loggers who worked on the river.