Idol touched on one issue, which is that the river is not navigable. But more importantly, the river is the main source for drinking water for about two million people, and is also used for agriculture. The North Saskatchewan River, seen in that photo of Edmonton (as well as my new place of employ!) is about another two million. As it is now, both are essentially glacial melt, and building any property near, over, or under them is a major regulatory hurdle at multiple levels of government, which wasn't so much an issue in cities developed prior to the 20th century when we discovered the environment, or where there is already significant motor-vehicle traffic on the rivers themselves. With respect to railways, it has more to do with the favorable terms given to the Canadian Pacific Railway in the late 19th and early 20th centuries. The development of Western Canada was an incredible undertaking; whereas all of built-up Eastern Canada has ready access not only to each other but to Europe and the world via the Saint Lawrence River and the Great Lakes system, access to the West was an industrial project. As such, the Canadian Pacific was given vast authority to secure land and rights to land, to police its own property, and to collect rent on abutting property, in order to pay for the tremendous cost of a transcontinental railway (through a harsher climate than the United States, with a much lower population to support it). The railway was the de facto government in much of Western Canada in its early development (a position they assumed from a different company, the Hudson's Bay Company, and not a government agent). Railway air-rights are to this day a huge issue in the urban development of Canada which the railways fiercely protect, as well as their land rights (the development of the Rail Lands on Toronto's lakefront is an excellent case study). Building a bridge or tunnel over or under railway land requires that railways consent, and often rent too. The handful of underpasses that cross the CPR mainline in downtown Calgary are on and under land leased from the Railway to the City via the Province. The benefits of collecting businesses in one location (eg: being able to send a bike messenger to collect a physical contract in 5 minutes) are tremendous, but fall off very rapidly. Rents meanwhile, fall off much more gradually. And so if a central business district is not going to be incredibly dense, you might as well just go to a campus-style pattern of development where rents are cheaper. I think that's a general trend in European cities, and all old cities really, to have a very large built-up area of medium density that still includes major offices and businesses, which dissipates the density in the central business district to a handful of financial interests that really benefit the most from physical distance. Part of that is age, since the modern skyscraper is a relatively new phenomena, and part of that is to do with the relatively flat river deltas of Europe being a fairly straightforward transportation problem.