Monday, March 30, 2015

Flight over the Driftless(?) Area

Being an enthusiast for both the earth sciences and aviation sciences, I always try to book my flights with a window seat, preferably behind the wing. That way, between watching the ground and the clouds, I get to watch all the flaps and ailerons working on the wing. I never get bored.

On a flight to Philadelphia the other week, I took a few interesting shots of the Mississippi River on the leg from Minneapolis to Chicago. Since it's such a short flight between the two, it didn't seem like we got too high before we had to start making our way down. Getting the chance to take some photos of this area was extra meaningful for me, as it's in the Driftless Area, which is where I first learned to be a geologist and a pilot.

Just out of Minneapolis airspace, eastbound, before we turned southeast. I wanted to get a shot of the Moon above the frozen Lake Pepin.

The formation of Lake Pepin is one of my favorite geology "stories". If you look on a map, the Mississippi River gets very narrow on the downstream side of the lake. This is due to the smaller, sediment-rich Chippewa River flowing into the Mississippi and dropping its sediment, causing a delta. This creates a natural dam, which then backs up the Mississippi River upsteam. This has two interesting effects on the local waterflow.

First, since the water backs up upstream, the river both widens (Lake Pepin) and the velocity drops (Q=VA, A increases, so V decreases). This creates a large, gently flowing area for boaters, which there are plenty of. Lake City is said to be the birthplace of water skiing. Due to the larger body of water, which also flows slower, the lake also remains frozen longer into the spring than the adjacent sections of the river, which has historically affected river boat traffic to the Twin Cities from more southern ports. The city of Reads Landing is so name because this is as far as ferries could go, and would have to land there, then take land routes the rest of the way north.

Secondly, the bottle-necking of the river from the delta causes the stream to narrow and flow faster (again, Q=VA, now the A decreases, increasing the V). This means that the section of the Mississippi River through the delta, in contrast to Lake Pepin, never freezes. This creates the perfect place for bald eagles to hunt for fish in the middle of winter. There are a number of spots along the river to stop and watch some eagles (link here). Thanks to places like these and efforts by groups like the National Eagle Center, the eagle population in this area has grown quite a bit, and has helped bring bald eagles off the threatened species list. I took this route along the river between Minneapolis and Winona countless times as an undergrad, and got to see firsthand the population of eagles in this area slowly increase.

A frozen section of the Mississippi River, southeast of Minneapolis/St. Paul. Note the deeply incised tributary valleys along the river valley.

North of a frozen Lake Pepin, looking south. The Mississippi River widens here due to the channel becoming constricted from an influx of sediment from the Chippewa River forming a delta bottleneck (on the left of the photo). Since the water backs up and slows down, it also stays frozen longer into the spring, delaying riverboat traffic into the Twin Cities above this point.

Flying over western Wisconsin. The incised stream valleys all retained a bit of snow despite the recent warmer weather. So much for being "driftless"(!). 

I zoomed in a bit on one of the rivers below because of all the oxbows. Everybody likes seeing oxbows from the air.

As we made a turn towards Chicago, the Mississippi River started to fall away from us, and I realized I wasn't going to get a chance to take a photo of my old stomping grounds near Winona, which is just out of view along the river.

In contrast/continuation of the above photo, here is one I shot of Winona while flying in to visit during the summer. Also, a bit lower than an airliner!

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