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Natural controls on internal drainage of sloughs

Whether or not your slough will drain quickly is a more complicated question than it appears

Many farms in Western Canada are plenty wet, and the 1.25 inches of rain we got on May 7 set seeding back a few days.

When a rain like that comes we hope for a hot, dry wind to “dry it up.” But, evaporation is a small party of the overall equation. Sloughs do not “dry up” as much as they drain. Or not!

We all know that sandy soils let excess water pass through quickly and the sloughs dry up quickly. Those with Henry’s Handbook can check page 140 to see a sandy slough near Saskatoon that drained away all spring snow melt water in 17 days.

Clay soils pass water much slower so we expect the sloughs to dry up slowly.

But it is much more complicated than that. Water moves more slowly through clay, but the aquifers beneath and the low water level in the aquifers is a big factor in slough drainage.

Figure 1 (at top of page) shows a slough in clay soils on University of Saskatchewan land. The photo was taken from Preston Avenue north of College Drive, for those who frequent Saskatoon.

Spring snow melt was early this year so by early April the slough was full and overflowing onto Preston Avenue.

The frost delayed the early entry of water but by mid-April the area was shrinking a little each day. By April 28 the slough was dry. On that day I measured the 10 foot well (figure 2 below) that had been installed in August 2016. The water table was at 3.02 m.

Figure 2. A closer view of the well site and slough center on April 28, 2017.
photo: Les Henry

The message is that even in a high clay soil, internal drainage of excess surface water can be fairly rapid — provided the water has somewhere to drain. There must be a downward pressure gradient to allow downward drainage to proceed. At some depth there must an aquifer with low head because of drainage to the South Saskatchewan River. That low head provides the mechanism to allow slough water to drain relatively quickly.

At this site there is about 15 feet of lacustrine (glacial lake) clay overlying glacial till deposited by the ice. No deep exploration has been conducted at this site but there must be a deep internal drain to facilitate the relatively rapid internal drainage of the sloughs. It is quite clear that internal drainage is controlled as much by the deep geology as by the near surface soil texture.

We must also consider that Mother Nature has been working at this internal drainage project for 10,000 years. It would make sense that some preferential pathways have been developed in that long time frame.

Very highly leached (white) sloughs

In the Black soil zone it is not uncommon to see sloughs that are white as a sheet. In those cases the white colour is <not> salt but rather an indication of the low organic matter content. The white sloughs would have been poplar bluffs with leaf litter instead of grass and hence the low organic matter.

This past winter I have scanned about 2,500 slides from my years at the University of Saskatchewan. Some were taken by me and some left with me when colleagues retired or left for greener pastures. Many of the landscape/soil colour slides were from summerfallow fields. Those slides are a precious resource that must be retained for future generations. We all hope that there is no backsliding to millions of acres of summerfallow every year.

In those slides there are examples of white, highly leached sloughs, in the Meadow Lake, Blaine Lake, Waldheim and Stockholm, Sask., areas to name a few.

Figure 3 (below) shows one such example near Meadow Lake. Even in 1993 summerfallow was a rare item at Meadow Lake so I was anxious to take a picture of something we may not see again for a very long time. Gleysol is a soil classification term referring to a poorly drained soil. I always argued with that definition. Theses soils are very well drained internally — they just have to let a lot of water pass through that collects because of poor surface drainage.

Figure 3. White, highly leached sloughs northwest of Meadow Lake. This photo was taken in May 1993.
photo: Les Henry

It is of interest to note that these white sloughs are almost always very high in soil test phosphorus. Because of the wet spring conditions in sloughs they can be cold and still respond to a bit of P with the seed. So, be careful in how you approach sampling these special soils.

Next time we will look at surface indicators of water coming out of the ground.

About the author


J.L.(Les) Henry is a former professor and extension specialist at the University of Saskatchewan. He farms at Dundurn, Sask. He recently finished a second printing of “Henry’s Handbook of Soil and Water,” a book that mixes the basics and practical aspects of soil, fertilizer and farming. Les will cover the shipping and GST for “Grainews” readers. Simply send a cheque for $50 to Henry Perspectives, 143 Tucker Cres., Saskatoon, Sask., S7H 3H7, and he will dispatch a signed book.


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