If you’ve looked at older maps of the Mississippi River and the surrounding Delta, you know the river has changed course many times since Spanish and French explorers began mapping the region.

Scientists are finding some of the old river channels may be providing more recharge capability for those locations in the underlying alluvial aquifer while some other river channels may not.

“In addition to looking at (the Mississippi River), we also flew several other river channels,” said James Rigby, research hydrologist with the USDA Agricultural Research Service in Oxford, Miss., discussing new instruments the U.S. Geologic Survey is using to peer down into the aquifer.

“You’ll recognize these. In the Cache River, we’re seeing something that’s pretty surprising,” said Rigby, a speaker at the annual Arkansas Soil and Water Education Conference at Arkansas State University in Jonesboro.

“About 15 to 20 meters below the (Cache) river channel we’re seeing this green and blue layer, which is finer sediments, silts and clays that would potentially be limiting the recharge from those rivers. Those are completely invisible unless we do a flight like this.”

Old river channels, on the other hand, may be creating a different scenario for recharging the Mississippi alluvial aquifer, which has been declining since farmers began irrigating more and more of their crop acres two decades ago.

Pilot project

Rigby is conducting a Managed Aquifer Recharge pilot project using bank infiltration from the Tallahatchie River as an injection source. The project is located in the Shell Mound area in Mississippi.

“Over in the west, it appears to be all silts and clays,” said Rigby, referring to a computer-generated map of the test area at the 5- to 10-meter depth. “We don’t think we get much recharge directly from precipitation at all in that area.

“But, in the east in the Tallahatchie Basin, we saw a lot more conductivity between the surface and the aquifer that’s driven by the scroll from the evolution of the basin. On the right, this is down at 35 to 40 meters, and on the eastern portion of that, you can see we’re already getting into those clays at the base of the aquifer.”

Scientists were surprised to see an old channel feature appear in another portion of the map of the test site.

“This looks like a paleo-channel from the ancestral Mississippi River basin,” he noted. “And one of the big differences is if you look down here at this cross section the aquifer, the connective portion of sediments bearing water, is maybe 30 to 40 meters deeper in that portion than it is even a few kilometers away.