Before logging this as found, cachers must read the lesson, perform the required tasks, and answer the questions below. Answers can be submitted by e-mailing the cache owner through his profile page or the Message Center. This earthcache has been placed with the permission of Manitoba Conservation and Water Stewardship.
To complete this earthcache you will need to hike the West Quarry Trail, a round-trip distance of 3.5 km on mostly level ground. This earthcache will bring you to three locations that highlight the uncommon physical geography found in this part of Hecla Island. No bushwhacking is required. My wife and I completed the round trip on foot in about 90 minutes, even while spending time at all three waypoints and reading every interpretive sign along the trail.
Shoals, sometimes called sandbars, are formed by the action of waves interacting with the sandy shoreline. Waves travelling across open water maintain a relatively constant wavelength (the distance from the crest of one wave to that of the following wave). As they travel over lake bottom that rises quickly, the wavelength shortens while the amplitude (wave height) increases, and the waves start breaking. This effect is called wave shoaling. It most commonly occurs near the shoreline, but can occur elsewhere in a lake as well. Waves lose their forward momentum at the shoreline, and as the water retreats back toward the lake it creates an undercurrent travelling opposite the wave direction. This undercurrent takes sediment from the shoreline and drags it into the lake. As the undercurrent slows, it can no longer carry the sediment and deposits it, usually where the undercurrent interacts with the incoming breaking waves. Over time this process builds up a deposit known as a shoal. Since the waves and corresponding undercurrent travel perpendicular to the shore, the shoal builds up roughly parallel to the shore.
As the shoal increases in size, it can induce more wave shoaling. In a nutshell: shoaling waves cause shoals to form; and shoals in turn cause waves to shoal. Stay with me.
Given enough time, shoals can extend to such a length that they cut off the shoreline behind them. The water within this newly enclosed area slows and eventually stills as it stops being fed by the larger lake body. The still water has little to no capacity to carry sediment, so the sediments settle to the bottom, increasing the clarity of the water relative to the lake. This shallow, smaller, and clearer body of water is called a lagoon. With plenty of water, but little movement, the lagoon booms with a diversity of life forms. All manner of aquatic organisms flourish. Wetland plants grow near the shores. Birds and insects find food and habitat. Animals come to drink the water and feed on the abundant plants. Other animals come to feed on the animals feeding on the plants.
Both the lagoon and the shoal can be observed from stages one and two. At stage three, you can observe something else.
A spit is a narrow extension of land protruding into the lake. Like shoals, spits are formed by deposition. They form in locations where the coastline (or shoreline, in a large lake) changes direction away from open water, with a turn greater than 30 degrees. Recall that shoals form as a result of waves hitting them perpendicularly (i.e. at a right angle). With spits, the waves hit the shore at an oblique (i.e. non-right) angle. Rather than draw sediment straight back into the lake, the water carries sediment a little further downstream, by a process known as longshore drift. Rather than form parallel to the shore as a shoal, a spit tends to form in a zigzag pattern.
The distal (farthest from shore) end of a spit may be hooked, due to a change in direction of waves or wind, or because of wave refraction. The spit at stage three is recurved in such a manner. As the spit extends, the water behind it is partially protected and slows, much as it does behind a shoal. In ocean bodies, this saltwater becomes brackish (not quite freshwater, but no longer as salty as before) due to the water settling.
Sources
https://en.wikipedia.org/wiki/Shoal
http://worldlandforms.com/landforms/sandbar/
https://en.wikipedia.org/wiki/Lagoon
https://en.wikipedia.org/wiki/Spit_(landform)
http://science.kennesaw.edu/~jdirnber/oceanography/LecuturesOceanogr/LecGeology/LecGeology.html
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To log this earthcache as found, first e-mail me the answers to the following questions (don't post them in your log!). You may log it as found before hearing a response from me, but logs with missing or poor accompanying answers to the first five questions may be deleted, and the cacher notified as to the reason(s) why.
1. The still waters of a lagoon allow wetland plants to flourish, creating excellent habitat for moose. Name three plants identified by the signs at stage one as good moose food.
2. At stage two you can stand on the shoal between Lake Winnipeg and the lagoon. On the day you visit, how high would the lake have to rise to spill into the lagoon?
3. Over time the action of water and ice has created levees in the shoal – “waves” in the way the rocks are arranged. How many levees can you identify? Also, observe the rocks that make up the barrier.
4. At stage three you will get to walk on a spit. Depending on the height of the lake, you may only be able to walk a short ways, or perhaps well offshore. How similar or different are the rocks on the spit from those that make up the shoal?
5. Examine the satellite photo below. Given what you know about the role waves play in creating shoals and spits, which direction must the waves have been coming from to produce these landforms where they are? (Hint: North is straight up in the photo.) This question can be answered at your computer if that’s easier.
6. (Optional) Post a photo of yourself and your GPSr as far out onto the spit as you are able to walk.
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