This EarthCache is one of several in an EarthCache Trail that runs along the Missouri River from the Chouteau County Fairgrounds Campground and Canoe Launch (Fort Benton - RM 0) to Coal Banks Landing (RM 41.5). This EarthCache is only accessible from the river. For more information and the first EarthCache in this series, please see the following link: click here.
For the next EarthCache in the trail, click here.
There is also a series of EarthCaches for the middle section of the river, from Coal Banks to Judith landing and for the lower section of the river, from Judith to Kipp Recreation Area. For this middle section EarthCache trail, click here.
For this lower section EarthCache trail, click here.
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Bank Erosion
For this EarthCache it is your decision whether you would like to exit your vessel at Evan’s Bend Primitive Boat Camp or float by as you learn. Good river lingo to know: ‘river right’ and ‘river left’ refer to the right or left side of the river, always when you are looking downstream. What you are looking for is along the right side of the river bank (terrain on each side of the river). You are looking at a type of erosion going on in the Upper Missouri River Valley. Geologically speaking, erosion is the gradual break down of earth materials by the action of wind, water, and other natural agents. Rivers erode land to create valleys. The Missouri River has been eroding this landscape for millions of years, though the outcroppings of rock you see as you float downstream are older than the river and valley.
River erosion processes (coolgeography.uk.com)
Cut bank erosion and point bar deposition as seen on the Powder River in Montana. (USGS)
Stream load, attrition, abrasion, down cutting, undercutting, cut banks, point bars, and sloughing are all terms used to describe river erosion processes. Stream load refers to the sediment carried in a river or stream. River channel size, how fast the water flows, and the amount of slope in the landscape the river flows through are all dependent on the amount of sediment a river can carry. Attrition (sediment particles hitting each other and breaking down) and abrasion (larger particles hitting the banks and bottom of the river) work over time to erode a river’s banks and bed (bottom). Down cutting refers to erosion of the river bed as abrasion breaks down the river bed over time. Undercutting is an erosional process by which abrasion erodes banks of softer rock below harder rock. This can create waterfalls and eventually removes support for the harder rock above. Cut banks and point bars respectively refer to the erosion and subsequent deposition as a river meanders (flows over gently sloping land and forms many curved bends). Take a look at the picture above and notice that erosional cut banks form on the outside meander of the river where the water velocity (speed) is faster. Point bars form on the inside of the meander where stream load is deposited and water velocity is slower. Sloughing in geology is a term referring to the slumping and shedding of soil, in this case at the river bank.
It is important to remember that erosion can also be caused by humans and animals. Animals approaching rivers and streams for drinking water and migration can cause banks to slough and slide. You may notice downriver a few noisy pieces of machinery in the river just by the bank. These are installed by the private landowners and are pumping water from the river to use for irrigation. Landowners also control rainwater, groundwater, and irrigation runoff along river banks by installing pipes that direct water into the river. Today, these manmade objects are built to have the least amount of impact on the surrounding environment but still have the potential to cause further erosion.
To claim this cache: Answer the following question and send the answers using Geocache's messaging tool.
Q: What geologic term describes what’s happening to the banks at Evan’s Bend Primitive Campground? (Hint: the bank is slumping and losing soil)