It is very difficult to imagine these floods. At the
flood’s maximum, more than 9 cubic miles of water roared past
a given point in the valley every hour. “The flow was
equivalent to 10 times the combined flow of all the rivers in the
world today, or 1,000 times the flow of today’s Columbia
River”, which drains the Columbia basin.
As the depth of the water in Lake Missoula increased, the
pressure at the bottom increased enough to lower the freezing point
of water below the temperature of the ice forming the dam. This
allowed water to seep into minuscule cracks present in the ice dam.
Over a period of time, the friction from water flowing through
these cracks generated enough heat to melt the ice walls and
enlarge the cracks. This allowed more water to flow through the
cracks, generating more heat, allowing even more water to flow
through the cracks. This feedback cycle eventually weakened the ice
dam so much that it could no longer support the pressure of the
water behind it, and it failed catastrophically.
The flood waters carried a huge load of gravel, cobbles and
boulders as it rushed through the Spokane Valley, which has been
known to cause the Spokane Floods and the channeled scablands of
Eastern Washington. Once the water started to subside the gravel,
cobbles, and boulders were left in the valley. The finer minerals
like sand and silt washed far down stream, leaving a relatively
clean layer rocks behind. If you look around this site you will see
some of the large boulders moved by these floods.
Covered by a thin soil layer, this very coarse, highly permeable
rock layer is now filled with water. Today, this underground water
source, the Spokane Valley-Rathdrum Prairie Aquifer, is a protected
"Sole Source Aquifer", the only source of drinking water.
Geologists estimate that the cycle of flooding and reformation
of the lake lasted an average of 55 years and that the floods
occurred several times over the 2,000-year period between 15,000
and 13,000 years ago. Alternate estimates for the peak flow rate of
the largest flood include 17 cubic kilometers per hour and range up
to 60 cubic kilometers per hour. The maximum flow speed approached
36 meters/second (130 km/h or 80 mph).
Resources: Signage at the posted coordinates.
Suburban Emergency Management Project
(http://www.semp.us/publications/biot_reader.php?BiotID=429).
Wikipedia-Missoula Floods
(http://en.wikipedia.org/wiki/Missoula_Floods)
To claim
this Earthcache you will have to answer the following questions and
complete two assignments. Please email me (AmplePeach@gmail.com)
the answers. Please do not post answers in your log, If answers are
posted, your log could be deleted without warning. Sorry, but this
is to make the find fair for all.
1. Visit the above coordinates to start
your lesson.
2. Look around you (River Bank) and
explain why you think these "Pebbles" and "Boulders" did not make
it farther down the river.
3. Estimate the distance across the
river.
4. Please post the time you started
this Earthcache along with the weather conditions.
You must now complete the following assignments, both
are required!
Assignment #1 Determine the flow rate of
the river at the time of your visit. This may be done by measuring
& marking a distance along the river bank, then tossing in a
stick upriver from your starting point. With a stop watch, time how
long it takes your stick to complete your measured distance course.
Then calculate that into "Feet Per Second" for the river (Distance
divided by Time). Once you have your answer please
(Click Here) to determine an approximation on how fast the
river is going in Miles Per Hour (MPH). Provide your answer in your
email.
Assignment #2. Visit the Geological
Feature (N 47.40.715, W 117.12.544). Your task is to determine the
volume (Cubic Feet) of this feature. You can do this by multiplying
the length x width x height. Explain how you think this feature got
to its current location. Provide your answers in your email.
Remember while reading above it was estimated the maximum flow
rate was 80mph when the flood came through here. After completing
this assignment you will have an idea of the difference between
today and then.
Although not a requirement a photo of
yourself near the site would be appreciated. :)
Please note that I will not be
responsible for personal injury or damage to your equipment. Please
be careful when you are near the river. Thank
you!