The “Island” is not really an island from an accurate technical standpoint but it is almost surrounded by water. Island Park is a well kept city park that boasts many features and activities including camping, golf, tennis courts, swimming pool, playground, barbecue and picnic areas, to name a few. Construction is now underway on a new sports complex on the Island as well. Plan on spending some time enjoying what the city and park has to offer. The city of Portage la Prairie is fortunate enough to have a natural gem in its midst, an oxbow lake along with an "island", separated from the city by a channel of water.
The “Island” is actually the cut off inside of a river meander. When a river flows over a nearly level flood plain, such as the Assiniboine does here, and the current of the river becomes slow, the river will meander, wander from side to side, looking for the easiest way to get to its end lake or ocean. Both mechanisms of erosion and deposition are involved in a meander. Erosion will take place on the outside of each bend because of greater water turbulence and faster flow and the material is carried away and deposited where the flow slows down, often at the inside of the next bend or meander. As the river continues to swing from side to side, the meander continues to grow from erosion on the outside of the bend and deposition on the inside. Logically, since a meander is eroded more on its downstream side than on its upstream side, the entire meander tends to move, over time, in the downstream direction of the river. When the narrow portion of land between the two curves of a meander is finally eroded through,(See the Chute in the figure below), either by slow erosion or by a flood, a new straighter river channel is made and an abandoned meander loop, called a cut-off, is formed. Then, when deposition finally seals off the cut-off meander from the main river channel, an oxbow lake is formed.
The picture below, compliments of Google Earth, shows the size of the oxbow that formed Crescent Lake and you can see that it is quite large. You can clearly see how close the beginning of the meander loop was to the end of the meander loop. Another cut off loop is clearly visible just downstream a few kilometers from Crescent Lake.
To log this Earth Cache you must post a picture of yourself and / or your GPSr at The Old Windmill, which is just down the road from the boat launch, as well as answers to the following questions. Do not post your answers in your log
1. Park in the parking zone at N49 57.768 W098 17.154 (Waypoint 1). Using your GPSr, measure the elevation where you park, then walk down to the nearby boat launch and measure the elevation at the water’s edge. Email me the two elevation readings and the difference in elevation. If your GPSr is incapable of measuring the elevations, then simply estimate the elevation difference from where you parked to the water’s edge.
2. The area of the Island can be approximated by a semi circle whose radius is 3.5 times the distance (as the crow flies) from the boat launch to The Old Windmill. Using your GPSr, measure the distance from boat launch to the old windmill and calculate the area of the Island.
3. If you travelled by boat from one end of the oxbow lake to the opposite end, you would travel approximately 6 Km. Use your GPSr to measure the distance from the boat launch to N49 57.837 W098 16.913 (waypoint 2) on the opposite shore at a bearing of approximately 95 degrees from the boat launch. This distance would be a good average width of the oxbow lake. Now, assuming that the lake has an average depth equivalent to 1/2 the elevation difference from the water edge to where you initially parked, calculate the volume of water in the lake in cubic meters.
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