I am by no means a "scientist", but I will go by what I have observed as a human being living in Calgary. Granted, I have only been in Calgary for 8 years, but this winter had to be the worst I have seen so far as snow is concerned. Spring didn't really come until June and the weather was on/off for weeks on end. Having no real history for me to personally go on, other than the big floods of 2005, I just think that the snow melt was a little more gradual. We had warm days, followed by cold ones, even more snow, going late into May and early June. This likley caused the "run off" to be gradual over time as opposed to getting a big flood all at once because the warmer weather was inconsitent until the snow was already all gone. In the city anyway.
I have no idea what the mountain streams were like last year as this is my first full season of fly fishing, and I only made it out side the city to fish twice last year, so I have nothing to go on. But if the weather was like this in Calgary, the higher altitudes in the mountains likley kept it cooler than normal and fluctuated the temperatures there as well. No real heat waves this year yet, so the snow didn't really melt all that quick. What run off and debris we did have was due to FINALLY getting some consistently warmer weather. The snow was mostly gone by that point anyway, so it could explain why the water level changes are so insignificant, because the "run off" was more gradual.
Again, I'm no scientist, but that's what I think might be going on.