This animation by NOAA shows the interface between freshwater (blue) and saltwater (green) where river outflow meets the ocean. It’s a geographic feature called an estuary. Because freshwater is less dense than saltwater, it flows downstream (right to left) riding above the wedge of saltwater. Eventually it mixes with the ocean brine.
How far the saltwater wedge penetrates upstream is a function of many factors. River slope and flow rate are the primary determinants. But high ocean tides and storm surges can push the wedge farther upstream. This has happened with the Mississippi River, where saltwater contaminated groundwater supplies during some of the recent hurricanes. The problem, of course, is exacerbated by rising sea levels. The US Army Corps of Engineers has actually built underwater barriers (called “sills”) to impede the saltwater wedge:
Estuaries are typically thought of as being on continental coasts, where freshwater meets saltwater, but there exist similar features called freshwater estuaries found on the Great Lakes and other large inland lakes around the world. The tides on inland lakes are only a couple inches. But with wind-driven surges, or seiches, water level changes can actually reverse the flow of water in an estuary.
A good friend of mine wrote about seeing seeing that phenomenon in his recent book about angling: You Shoulda Been Here Last Week. And it happened on the East Twin River in the town we both grew up in. He often fished that river so was in tune with its rhythms. I was aware of the distinction between a river and an estuary, but didn’t know the current near the mouth of a river could reverse under certain conditions. That led me to some research that I present in this post.
Freshwater estuaries on inland lakes (like the East and West Twin) don’t have to contend with a saltwater wedge. Rather, the situation is reversed: The river water is carrying sediment, biomass, dissolved road salt, and agricultural runoff. That makes it more dense than the lake water. You can see the plume from the rivers, which combine at the harbor, extending out into the lake:
Here’s a closeup:
This is the normal behavior of the rivers. But as my friend wrote, he saw the East Twin reverse flow on more than one occasion. That makes sense. The rivers don’t have a huge watershed, 184 sq miles for the East Twin, and 180 for the West Twin. Plus the terrain has a shallow slope, so they only flow rapidly during heavy rains:
Those flow reversals were likely wind-driven events. I suspect it doesn’t happen often. Using the Historical Imagery feature on Google Earth, I looked at dozens of images taken over the years. None showed a blue estuary — which is how it would look if the current reversed.
I learned much about estuaries and rivers while researching this post. One surprising factoid: Studies have shown salt levels rising in Lake Michigan, from 1-2 milligrams per liter in the 1800s to over 15 milligrams today. The majority of this is due to road salt runoff contaminating watersheds.
Next Week in Sky Lights ⇒ OceanWell Desalination Innovation