This is not your usual type of geocache. For one thing, there is no physical geocache for you to find. Rather, this is a special type of virtual cache known as an EarthCache.
An EarthCache teaches an earth science lesson. The cache page must include logging tasks that help teach the same lesson. Geocachers must complete the tasks before they log the EarthCache as found.
For more information on EarthCaches, see the Geological Society of America's website.
Finding this Earthcache:
In order to get credit for finding the cache, it is not enough to log it below. You also must answer the following five questions and message or EMAIL your answers to the Cache Owner. DO NOT place your answers in your log entry!!!
NOTE: The logging tasks require a bit more thought and effort than many Earthcaches, thus justifying the higher D rating. However, perfection is not required. A good faith attempt to fulfill the logging requirements should be sufficient.
1. The given coordinates take you to the canoe launch site just below the Thiensville Dam. Using either your GPS unit, the Compass app on your iphone 6 or newer, or whatever equivalent app on your smart phone, what is the elevation above sea level of the surface of the Milwaukee River at this point at the time of your visit?
2. The average surface elevation of Lake Michigan is about 577 feet above sea level. Assuming that the distance following the river channel between your elevation reading and the mouth of the Milwaukee River is 20 miles, what is the channel slope in feet per mile for the section of the Milwaukee River between those points?
3. What impact do you think it would have on the Milwaukee River and its channel between here and its mouth if its channel slope was higher?
4. Why do you think it is important to measure the slope of a river or stream based on the length of its flow or channel rather than merely using the straight-line distance between two points on the river?
5. Go to WP2 by the Thiensville sign on the north/west side of the river off Green Bay Road (from GZ you can walk west on the Rotary Riverwalk to Green Bay Road and then south to the sign). Describe any differences you see between the north/west (near side) riverbank at WP2 and the south/east (far side) riverbank.
6. Continue further south on Green Bay Road to WP3, the canoe launch at Molyneux Park. Again describe any differences you see between the west (near side) riverbank at WP3 and the east (far side) riverbank.
7. What, in terms of water velocity and its effects, do you think explains the differences identified in response to tasks 5 and 6?
Finally: In addition to sending me a message or email with answers to the seven tasks above, post a photo with your online log showing yourself (face not required) or some item of personal property at GZ with the Thiensville Dam in the background.
Please message or email your answers to tasks 1-7 BEFORE you log your find, but there is no need to wait for a response from me before logging your find below. I will let you know if there is a problem.
Channel Slope
It is a basic principle of gravity that water flows downhill. While we generally do not think about it outside of waterfalls, rivers and streams must flow downhill from their source to their mouth. And sometimes, it is important to measure how steep that flow is.
Channel slope or gradient is the difference in elevation between two points on a stream divided by the distance between them measured along the stream channel. All other things being equal, the flow velocity (i.e., the water speed), and thus power of the stream to do work (turn a mill wheel, erode and transport sediment, etc.) is directly related to the slope of the channel. The steeper the slope, the faster the velocity of flow.
The slope of a stream effects the size and shape of the stream. For instance, channel patterns are influenced by the flow of streams and the accumulation of sediment.
Scour or riverbed erosion -- Where a stream flows down a steep slope, velocity will increase resulting in increased erosion of the riverbed (a process called scour) as the flow accumulates silt, debris, and any chemicals or pollutants contained in it. Where that stream then flows onto a gentler slope velocity decreases and suspended material that was carried along by the water will settle out as sediment in a process called deposition. As a result, the steeper slope is eroded away over time and the more gradual slope is filled in, making for a more uniform slope.
Bank or Lateral Erosion -- The same concept applies to the banks of the stream or river as well. A greater channel slope and faster flow will cause greater erosion of the banks. This is especially true on the outside bank of any sharper turns. As the water travels faster on the outside of the curve and slower on the inside, the faster water picks up sediment from that bank while the slower water on the inside of the curve deposits sediment there.
This process ultimately results in a straighter channel as the meanders (the S curves in a river or stream) are eroded away.
For more on this process see InternetGeology's page on Meanders.
If you want to get into a LOT more detail than is appropriate here on what all impacts velocity of stream flow, Google “Manning’s Equation.”
Calculating Channel Slope
Calculating channel slope for all or a given section of a river or stream requires three pieces of data:
A = the altitude or elevation above sea level of the starting point,
B = the altitude or elevation above sea level of the ending point, and
C = the distance in miles between the two altitude measurements along the flow of the stream.
Between meanders and bends and the like, few rivers naturally follow a straight line. Accordingly, it is insufficient simply to measure the distance between one point and another on the river. For instance, the straight-line distance between the given coordinates and the mouth of the Milwaukee River is about 15 miles.
Yet, the river itself travels about 20 miles, meandering back and forth between those points. You can get a more accurate calculation of the actual length of the river by using Google Maps’ Measure Distance tool and adding stages at each turn along the route of the river as indicated in the image below.
Another example is the nearby Mequon-Thiensville Fishway. You can find an informational marker on the Fishway north of the dam near the parking lot.
Water enters the Fishway from the river just upstream from the dam and flowing around for a while alongside the river, empties back into the river just downstream of the dam next to Ground Zero for this Earthcache. While water flowing over the dam ends up in the same place as that flowing through the Fishway, the latter flows a lot further to do so, meaning that its slope from the entrance to the Fishway to its exit is lower that than of the water flowing over the dam, making it much easier for fish swimming up river to spawn.
To calculate channel slope, begin by subtracting the downstream elevation (B) from the upstream elevation (A). Then divide the result by the number of miles between the two points. The formula would look like this:
(A-B)/C equals channel slope in feet per mile.
As an example, if the source of a stream is 200 feet above sea level and the mouth of that stream is at 100 feet above sea level, the difference is 100 feet. If the stream is 40 miles long, the channel slope from the source to the mouth would be 100/40 or 2.5 feet/mile.
That does not mean that the entire distance has a consistent slope. Different stretches of river can have different slopes. instead, we are calculating the Average slope over the chosen distance.
Permission:
Permission for placing this Earthcache was granted by Andy LaFond, Director of Community Service/Public Works, Village of Thiensville.