As water starts to run this time of year, it takes away more than the memory of winter’s snows: it also carries away a lot of soil, dumping it in the nearest ditch, which carries it to the nearest river, which carries some of it eventually to the sea.
Sometimes the water also finds its way through our basement walls, through cracks that we somehow didn’t notice last year, but this year seem to be getting wider and wider. And meanwhile, the streets are showing every sign of falling apart. What’s going on?
What’s going on is erosion, a process that eventually grinds even the mightiest mountains down to prairie.
Erosion is happening all the time: spring run-off just brings it a bit more to our attention. There are five main agents of erosion. Running water and wind are the two we’re most familiar with in Saskatchewan, but gravity, ice, and waves are also mighty eroders.
Gravity erosion, or “mass wasting,” is simply the slow downward creep of rocks on hillsides (Saskatchewan’s dearth of hillsides is why we’re not very aware of it here). Occasionally something happens to accelerate that process and a landslide occurs, but normally the process is almost imperceptible.
What forms loose rocks on the hillsides in the first place, however, and one of the first steps in all forms of erosion, is something we are aware of: weathering. Rocks may seem eternal, but rocks exposed to the atmosphere are anything but. Many rocks contain soluble materials that running water can leach away. Decaying vegetation can produce acids that hasten the dissolution of the rocks. Various chemical reactions can also break down rocks, some of their constituent elements binding with oxygen or carbon in the surroundings. And then there’s frost action.
Frost action is weathering process caused by repeated cycles of freezing and thawing. Water expands almost nine percent when it freezes. If it’s trapped inside a rock, or between particles of soil, or in a tiny crack in your basement wall, it exerts enormous pressure, widening the space it occupies. The next thaw allows more water to seep into the enlarged space; at the next freeze, that water expands again. The longer this freeze-thaw cycle continues, the bigger the opening becomes. Rocks can split, if the water is trapped in a crack, or shatter, if the water is trapped in pores, and the crack in your basement wall gets bigger and bigger, especially this time of year, when it can thaw and freeze every day.
The frost cycle also explains another Saskatchewan phenomenon that any farmer is familiar with: rocks keep finding their way to the surface. When groundwater freezes, it forms ice crystals that push upward. The ground under rocks and pebbles freezes first, because its sheltered from the sun, so the crystals that form under rocks start growing first and are therefore larger than the crystals that form in the surrounding ground. As a result, rocks are pushed up more than the ground around them, and eventually surface. Once they’re on the surface, they’re more susceptible to all the other forms of weathering. Unless a farmer comes along and moves them, they’ll eventually decompose into new soil.
But as weathering slowly grinds rocks smaller and smaller, they become susceptible to water and wind erosion. (Of course, enough water can move even the largest boulder, but fortunately such floods are rare.) Water erosion includes more than just moving soil and rocks from place to place: it also includes the chemical action of water that I mentioned when talking about weathering, which continues no matter how small the rock is, and the abrasive effect of rock particles in moving water, which is what polishes the smooth, rounded rocks you find in streams.
If the snow all melts off and we don’t get any rain for a while, we can expect to experience wind erosion. Saskatchewan gets its strongest winds in the spring, because of the temperature difference between the still-frozen land north of us and the rapidly warming land south of us. Strong winds are able to pick up rock particles as big as two millimetres in diameter, and most of the particles in topsoil (especially topsoil which has been exposed to the frost cycle, which continues to break down the soil particles just as it breaks down larger rocks) are much smaller than that. Enough grit blowing in the wind can also abrade or polish exposed rock surfaces, not to mention exposed paint jobs.
Ice erosion, caused by glaciers, isn’t something we see much of, but it shaped our landscape. Glacial ice freezes to rock fragments, plucking them loose from their mother rocks, and then grinds them along underneath, abrading and polishing bedrock. Softer bedrock is abraded more, creating depressions, which, when the glaciers retreat, fill with water to become lakes. Retreating glaciers also leave behind all the rocks they were carrying with them–which is another reason why picking rocks is such a popular spring activity for Saskatchewan farmers.
Finally, there’s wave erosion, which we see a little of in our larger lakes, where wind-formed waves splash against beaches and slowly tug the land down into the water. Obviously, wave erosion is much more pronounced along the ocean coasts, where in some places it has formed steep cliffs.
The flip side of all this erosion is sedimentation. All the material carried away by the various agents of erosion has to end up somewhere, after all. The rocks on hillsides eventually end up at the bottom of the hill. Minerals dissolved by water are precipitated somewhere, maybe forming stalactites or stalagmites in a cavern, or a brown crust inside your tea kettle. Soil carried away by water forms mud somewhere else. Soil blown away by the wind drifts into ditches or is washed out of the sky by rain a hundred miles away.
Erosion and sedimentation are vast natural processes that we’re subject to and, on a geological scale, can do little to control. On a smaller scale, we can protect soil by leaving vegetation or stubble on it year-round and directing the flow of water where we want it to go instead of leaving it find its own, often destructive, path.
As for that crack in your basement wall: well, my advice is, don’t wait too long to seal it.
Remember, you’re dealing with the same force that turned Saskatchewan’s primeval mountains into what you see today between here and Saskatoon.
Scary, isn’t it?
