Alluvial fans generally form where a river confined to a narrow canyon empties onto a plain where river water is free to spread in multiple directions. River water pouring down a canyon picks up considerable amounts of sand and sediment and during flood events can carry pebbles and boulders with ease. This material tends to be evenly distributed in an arc around the stream outlet.
Stokes Law
George Gabriel Stokes derived an expression in 1851, now known as Stokes' law, for the frictional force – also called drag force – exerted on spherical objects in a viscous fluid
The force of viscosity on a small sphere moving through a viscous fluid is given by:
Fd = 6·π·μ·R·V
where Fd is the frictional force – known as Stokes' drag – acting on the interface between the fluid and the particle, μ is the dynamic viscosity, R is the radius of the spherical object, and V is the flow velocity relative to the object. In SI units, Fd is given in Newtons, μ in Pa·s, R in meters, and V in m/s.
In short, this means that objects with a larger radius sink faster in water. This translates to large rocks being deposited closer to the mouth of the stream and fine silt travelling farthest from the stream mounth.
Two Very Different Alluvial Fans
Between Glacier and Waterton National Parks, there are two major fans, although every stream draining into Upper Waterton Lake has a small fan attached. Under natural processes, an alluvial fan will spread out in all directions and during a major flood, river channels will change drastically. These plains provide perfect conditions for wildflowers and grasses to grow, and in turn provides ideal habitat for ungulates, birds, and mammals. The material washed down from the mountains is well-drained and fescue grasses and aspen trees thrive under these growing conditions. Due to the wide open plain, wind prevents snow accumulaton and provides an easily accessible food source during the winter.
Blakiston Fan has been permitted to grow almost unhindered for centuries and is the largest of the two major fans in Waterton. The forces that create this fan are very powerful and over time have bisected the north part of Waterton Lake into middle and lower Waterton Lake. There is a road to the famous Marquis Hole swimming hole which periodically gets wiped out by floods, although this is a natural process. Fortunately there is no human habitation on the fan so it has been permitted to develop naturally.
The Townsite Fan was formed the same way from Cameron Creek and prior to human habitation would have looked identical to the Blakiston Fan. The two fans diverge when it comes to human intervention. In the past, spring floods have caused considerable flooding in the townsite. As a result, after Cameron Creek cascades down Cameron Falls, the creek has been channelized and rock armoured all the way to Waterton Lake. This effectively sets the destiny of the creek and ensures that it will no longer be able to meander across the plain such as at Blakiston.
To Log This Earthcache:
E-mail me the answers to the earthcache through geocaching.com message centre. If I do not recieve a response with the log in a reasonable amount of time, I will delete you log after contacting you.
Blakiston Fan
1.) Tell me how the gradation (gravel size) changes as you travel from this cache's coordinates and then down the road as far as you can go (see the waypoint for road turnoff).
2.) Get an elevation at the coordinates E1 and then an elevation at a point out on the road E2. From where you take E2 record how far you are from the cache's coordinates. This is D. Give me the slope of the fan (the longer the distance, the more accurate the slope).
(E1-E2)/D * 100 = Slope (in percent)
Townsite Fan
3.) Look at your feet and tell me the size range of the gravel at your feet. Try to find gravel that looks natural, not necessarily trucked in for the beach or carried straight down the river.
4.) Tell me three things that are different between the Blakiston Fan and the Townsite Fan.
This geocache is owned and maintained by a proud GEAA member.
GEAA provides general information about geocaching, encourages newcomers, and promotes the hobby of geocaching through fellowship, education, and advocacy with land managers and government.
As the geocache owner, I affirm that the image above links to a local geocaching group that is active in the community and contributing to geocaching in a positive way. The link has not been checked by Geocaching HQ or by the reviewer.