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Under Construction

The North American rivers in the menu at left have one thing in common - they all came under the  impact of glaciers.  All were either covered with ice or bordered glacial ice during the last glaciation or they were swept clean by floods that were caused by glaciers.  The most recent significant events happened about 12,000 to 15,000 years ago.  Thus, the ecological systems and aquatic communities are relatively young. 

Sources of information and brief comments are given below.


       Columbia and Willamette Rivers-------
 Volcanic vs. Sedimentary,
Floods and lakes.

Between 11 and17 million years ago,
thousands of square miles
in Idaho, Washington, and
Oregon were covered by
dozens of lava flows that
erupted from fractures in the
earth's crust near the
Lava flows travelled hundreds of
miles all the way to
the Oregon coast, and affected
the Willamette Valley and
basin of the Willamette River in
Most of the main channel of the
Columbia River  was
cut through these lava flows.

Basalts and lava flows
are seen in all 4 pictures in
the following pdf.

Oregon Gorge and Coast.pdf

Next of importance in this
discussion is that between
15,000 and 12,800 years ago,
more than 40 floods scoured
a large region between the
Montana-Idaho border for
hindreds of miles through
Washington and Oregon, the
Willamette Valley, and the ocean in Oregon. 
Walls of water more than
300 feet high repeatedly
flushed and  scoured the
channel of the Columbia
River and the floods several times
formed a lake in Oregon
in the Willamette Valley. The lake reached the city of  
Eugene at the south end of
the Willamette valley, the source
area of the Willamette River. 
Flood waters and floating ice
deposited rocks and
silt from as far away as
Montana and Canada in the
Willamette Valley of Oregon.
Glaciers from Canada
crept south, forming an  ice dam
across a valley near Sandpoint
Idaho. The result of the dam was a huge lake, Glacial
Lake Missoula.

Ice is ligher than water, and ice
does not form a stable and
permanent dam.
As the lake
level rose, the lake water  lifted the ice that formed the dam.
A huge and sudden flood resulted.  This happened repeatedly in the 12-15,000 year time period mentined above.

River  communities and ecolocical
systems were repeatedly eliminated and destroyed by the floods.  

The communities that exist now or existed in recent times are   relatively young.



Missouri and Saskatchewan River SystemsSedimentary, not volcanic,
glacial landscape
Beginning about 1.6 million
years ago, an up  to about
12-15,000 yeas ago,
glaciers that were
sometimes several thousand
feet in thickness advanced
repeatedly over what is now
Saskatchewan and the
Dakotas. Each glacial
advance diverted water flow
around the glacier edge, and the
modern Missouri system
now drains south rather than
north as it did in previous
times. The Missouri River in
generally marks the
farthest advance of  glaciers
in the Dakotas.
The Saskatachewan River
system was repeatedly
overrun and completely covered
by advancing glaciers. 
The book at left is a good
source of information on the
underlying bedrock of the
region- ranging from Pierre
shales of the late Cretaceous
(75 million years ago) to
25 million year old siltstone,
sandstones, and limestones
of Miocene age (for North Dakota).
No summaries or comprehensive
guidebooks have so far been
found for Saskatchwan, nor for
the MacKenzie system in Canada. 

Many maps and publications
are available on line at the
South Dakota Geological Survey
site --See Below**-
that give information on
glaciation, bedrock, drainiages,
and many other subject
relevant to the Missouri River in
South Dakota.

The book at left is excellent for
giving information on the same
topics for North Dakota. It can
be found by seaching on line.

** South Dakota Maps and Geology/Geography Publications (upper-bulletins, lower- maps)