Many many moons ago, I took a geography course and learned about this feature. This bridge is located over a glacial meltwater channel that drained glacial Lake Edmonton after the last ice age about 12,000 years ago. Beginning east of Nisku, the channel can be traced as it winds southeastward through a chain of lakes-Saunders, Ord, three unnamed ephemeral lakes, Coal and Driedmeat.
More on Lake Edmonton
As the thick ice sheet began to melt, a gigantic ice dam trapped the water. For almost 100 years, “Lake Edmonton” was walled in on three sides by sheets of ice. Water covered the area from Morinville to Leduc, from Stony Plain to Fort Saskatchewan.
When the lake disappeared, it did so in a flash of geological time. Water found a low point under the dam and began to flow south. “The force of the flow ate at the ice above and the ground below, rapidly enlarging this sub glacial drain until, in a torrent of water and ice and rocks and soil, the lake poured out of its icy basin and into the Battle River, carving a deep channel as it went.” (B. Huck and D. Whiteway)
One theory suggests the flood lasted only a few weeks. Imagine huge flows moving several thousand cubic kilos of rock and sediment! The flood set the stage for the appearance of the North Saskatchewan River. Water flowing over the old glacial lakebed eventually established a preferred channel. Erosion then formed the Edmonton River Valley system we see today.
(Sources: Atlas of Alberta Lakes, Coal Lake entry; http://www.larchsanctuary.ca/geology.html)