The last couple of posts have described fractured clay till and an overlying terrace gravel that store groundwater in the Creek pasture. Both aquifers “leak” out onto the ground surface to form springs. However, this post is about a layer of sand and gravel buried under the floodplain of the Creek. When water in the channel is high, water goes into the aquifer. But when the channel water is low, the groundwater from the buried sand flows out into the channel.
The buried sand isn’t a layer of uniform thickness and doesn’t seem to be present everywhere. This map shows the location of boulders (marked with red Bs) in the Creek channel. In these places, the buried sand is mainly missing. In contrast, the sand is locally exposed above the water at several places (shown by the black Xs). The dashed lines outline the extent of the aquifer and the blue arrow indicates a place where groundwater from the buried sand flows out into the oxbow which is an abandoned channel segment. The numbers are the locations of the next four photos.
This pair of photos illustrate the variable thickness of the sand layer. The top of the light colored sand is fairly close to the surface of the water in the Creek near the bridge. Similarly, the sand layer at the left of the second photo is also close to the water. However to the right, the sand is thicker and the top of the sand layer is much higher above the water. The buried bottom of the sand layer is probably also at different depths, so the thickness of the layer is highly variable.
Boulders are found in the channel where the sand layer is totally missing. The staff in this photo is about five feet long, so this is a big guy boulder. Not too far away, the sand is present and the top is easy to see under the tan stream deposits with dark soil development at the top near the grass. The red arrow points to an area where the sand is stained orange by groundwater movement near the base of the layer. So, we can see both the top and the bottom of the buried layer; it’s not very thick here.
This past spring, the water level in the main channel was high enough to flood the oxbow. These two photos show the inlet into the wetland on the south side of the oxbow in the left photo and the connection between the main channel and the pond on the north side of the oxbow in right photo. At these high levels, surface water was probably flowing out into the aquifer, especially out of the pond which is closest to the buried sand layer. In this condition, the surface water is said to be “recharging” the buried aquifer.
As the year progressed, we ended up in drought conditions. The wetland totally dried up(left photo), but the pond only got smaller(right photo). It didn’t dry up completely because it was getting support from groundwater flowing out of the buried aquifer. The wetland on the other side of the oxbow did not get the benefits of the groundwater.
Eventually, even the pond dried up because the aquifer got drained. The photo on the left shows the pond just a few weeks ago. You can see the positions of the edge of the pond as a series of small steps that document the progressive shrinking of the pond. Without the input from the buried aquifer, the pond disappeared. However if the aquifer can be filled again (“recharged”), groundwater could again flow out into the pond. The photo on the right was taken several years ago after the pond had frozen over. The white lobes of ice that extend out from the high bank and onto the gray ice represent groundwater plumes frozen in place as groundwater leaked out of the storage area. So, back then the aquifer had excess water that could help to maintain the pond.