Chris Pecora

Building Where We Shouldn’t

Flood lessons yet unlearned.

By Kevin Van Tighem

If you want to see elk or wolves in Banff or Jasper National Parks this winter, here’s a tip: look on an alluvial fan. Alluvial fans develop wherever a creek running down a steep drainage flows out into a larger valley. Over the centuries those creeks deposit countless tonnes of eroded rock, building fan-shaped landforms at the edges of larger river valleys. The big valleys our highways follow through the Rockies trap the flow of dozens of such drainages.

Mountains are constantly falling apart as their rock weathers and erodes. Gravity pulls all the loose bits downhill, where they accumulate on avalanche paths, talus slopes and valley floors. Tonnes and tonnes of loose material gradually fill those steep mountain valleys.

Winter snows and spring rains flush the smallest bits away when creeks flood each spring. Gravity gives floodwaters a lot of power. The faster the stream’s flow, the more rock and gravel it can move. Arriving at the valley floor those floodwaters lose momentum and have to  deposit most of their load. Those accumulated deposits are what form alluvial fans.

Once every few years much bigger floods pour out of those steep valleys. That’s when most of the stockpiled rock debris gets flushed down to the alluvial fans at the bottom of each drainage. During those big flood years, streams often change course completely as their former channels fill with new deposits. An alluvial fan is a mosaic of abandoned creek channels of varying ages, gradually filling in the mouth of the valley.

We’re starting to understand that when steep creeks flood it isn’t just water we have to deal with; it’s almost unimaginable amounts of rock and mud.

That dynamic, shifting nature is why alluvial fans support rich patchworks of vegetation—from old spruce stands, through young poplar and willow thickets, to grassy gravel flats near the active stream channel. That diversity of vegetation is one reason why wildlife like alluvial fans.

Years ago I worked on the original wildlife inventories of Banff and Jasper National Parks. We counted wildlife from frogs to bats, songbirds and bighorns to determine which habitats were most important.

When we tallied up our results at the end of six years, we found that alluvial fans and stream floodplains were by far the most productive wildlife habitats. Not only are they critical range for wintering elk and deer and the predators that feed on them, they are essential year-round for rodents, songbirds, bears and other wildlife—far more important than adjacent forests and mountainsides.

But their gentle slopes and well-drained soils make these sites irresistible for developers too. The town of Banff sits on parts of two alluvial fans. Almost every other alluvial fan in Banff’s Bow Valley has a road, campground, motel or other development on it. Jasper has fared better—only a few of the fans along the broader Athabasca Valley have been developed.

Good land-use planning would have left those alluvial fans for wildlife and put our infrastructure on stable landforms such as the glacial terraces that line many mountain valleys. But developers and planners had the benefit of neither wildlife inventories nor bitter experience in the early 20th century when key decisions were being made.

The 2013 flood certainly provided the bitter experience. Those steep mountain valleys had been accumulating stockpiles of broken mountain for decades, awaiting the big flood that would flush them down to the alluvial fans. When that big flood arrived, our stuff was in the way.

Canmore—my hometown—had spent three decades covering the alluvial fan of Cougar Creek with housing. That wiped out some of the valley’s best wildlife habitat; one neighbourhood is even called “Elk Run.”

Town planners thought they could protect those houses by confining the creek to a single drainage channel. But steep mountain creeks aren’t just water when it floods—they are water, mud, gravel, boulders and trees. Cougar Creek filled the engineered channel with rock debris from its steep upper valley and washed away the banks so it could cut a new channel. Because that’s how alluvial fans work.

The repair costs in the Bow Valley alone will exceed $100-million. Mostly, those repairs have involved putting the creeks back where we want them. But these are alluvial fans; the creeks won’t stay there. Engineers and civic planners are starting to understand that when steep creeks flood it isn’t just water they have to deal with; it’s almost unimaginable amounts of rock and mud. But their solutions still seem to involve turning a dynamic piece of landscape into a static one.

That won’t work. The mountains are still falling apart and those steep valleys continue to stockpile all the debris high in the headwaters, waiting for the next flood to flush it out. That flood will come. Mother Nature always wins. All we can do is move our stuff out of places that are meant for wildlife.

Kevin Van Tighem’s latest book, Our Place: Changing the Nature of Alberta, was released in spring 2017 by RMB.


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