In 2018 and 2019 we had record total rainfalls, but this year it’s been dry. That gives us a window on how water is stored underground and how that water interacts with vegetation, topography, and surface water in sloughs, springs and the Creek channel. Underground water (“groundwater”) is stored in tiny holes between grains of sand or gravel or in narrow openings in fractured clay or bedrock. In the karst country of southeast Minnesota there are caves and caverns that have actual underground streams of open water. But, in southwestern Minnesota, groundwater is stored in tiny pores or narrow fractures (“pore spaces”) found in layers of different geologic material (“aquifers”).
There are at least four aquifers along Kanaranzi Creek. Bedrock aquifers are deeply buried hundreds of feet below the land surface and information about them is limited. However, there are three distinctive water-bearing layers located at or very near the surface as shown in this sketch. The oldest is fractured gray clay with incorporated boulders that was deposited directly by glacial ice (“till”); it forms the hills and uplands along the Creek. The till is capped by wind-blown silt (“loess”) that also covers the intermediate age aquifer. That second aquifer is a sand and gravel layer deposited by glacial meltwater “(“outwash”); it underlies a flat terrace that stands just above the Creek. The third and youngest aquifer is a discontinuous layer of sand and gravel outwash that’s buried under stream sediments beneath the Creek’s floodplain.
We’ll look at the middle aquifer first because it’s characterized by distinctive vegetation patterns. Groundwater seeping out along the edge of the flat terrace is marked by patches of dark green plants. These are basically springs that might be used for livestock if they’re modified somewhat. My mother’s father used to talk about building a house near there because of the good water source. Although, it might not be too reliable as a domestic water source, it does contribute to wet conditions in row crop fields.
About thirty years ago, drainage tile was installed in the cropland and the water flows down into a wetland on the floodplain below the terrace. It’s sort of hard to see the tile water through all the brown grass in the left photo. But, the right photo shows the dark green plants that grow around the springs at the terrace edge are distinctively different than the vegetation in the paddocks on top of the flat terrace surface or than the crops on the adjacent field. The water from the drainage tile helps to sustain the wetland at the base of the hill.
Where the main channel of the Creek gets close to the terrace, it erodes into the steep bank to give us a view of the geologic layers. The aquifer is sandwiched between the overlying wind-blown loess and the underlying glacial till. Black soil is developed at the top of the buff-colored loess and the light colored sand and gravel outwash lies on top of the glacial till.
In places, chemical reactions in the groundwater produce a cement that sticks the sand grains together to make a sandstone, but in most places the sand and gravel is loose and uncemented. Elongate cigar-shaped cemented areas may be an indication of groundwater flow directions. The boulders near the water are lag deposits from the till after the clay has been carried away by the flow of water in the channel. Those big rocks were probably not carried by the Creek, but rather are all that remains of the glacial till after all of the clay has been removed.
These last comments are speculative interpretations, but most of this post describes data that anyone can see. The older glacial till and the younger outwash buried beneath the floodplain are not as easily observed as this intermediate-aged terrace outwash. The cartoon at the beginning of this post is a “guess” about what’s underground. If we had data from test holes drilled every few hundred feet across the landscape, it would be a much more reliable interpretation. The next two aquifers are much harder to “see” and interpret, but the next several posts will look at them in more detail.