The three forces of nature that shaped this
landscape to become mountains and valleys are the
shifting of the land plates to push the mountains up,
two major volcanic eruptions and finally the massive
Bonneville Flood. Several rivers and streams ad their
courses altered by these natural events, but virtually
all the lakes and reservoirs came into being at the hand
of man to fill the need for crop irrigation andelectric
power.
Looking at a geographic map of the United States
people wonder how Idaho’s eastern and western boundaries
were decided in such irregular pattern. The northern and
southern borders of the state are nice and straight
lines by contrast. The simple explanation is the western
border used to be the coastline of the Pacific Ocean
until California, Washington and Oregon rose up from the
ocean floor pushing Idaho upward to become the western
side of the Rocky Mountains. The highest point of the
Rockies became the Continental Divide which is defined
by where the snowmelt runoff from these mountains goes.
This also explains the border of Idaho and Montana.
Montana’s water runs southeast to the Missouri and
Mississippi rivers to the Atlantic Ocean. Idaho’s water
runs northwest to the Columbia River and the Pacific
Ocean.
The Bear River is part of the
westward flowing slope of the Rockies. It starts in the
mountains of Utah and flows northwest to Idaho’s Bear
Lake Valley and on to Soda Springs where it turns south
through Preston and empties into the Great Salt Lake,
which doesn’t make much sense. It travels over 400 miles
to cover a distance of only 90 miles. It used to flow
through Pocatello to the Snake River and onto the
Pacific Ocean.
Left - Fishing on the Bear River near Preston
Then a rift volcano still visible near
the intersection of state highway 34 and US 30 near
Soda Springs raised the valley floor and sent a wall
of molten lava 40 miles west to Pocatello. A
second major flow occurred shortly after following the
same path. These lava flows turned the Bear River
southward. The remnants of these flows are spectacular
to see at Grace, McCammon, Inkom and Pocatello.
Right - Lava flow near Inkom
The final event affecting the geography of Southeast
Idaho occurred just 15,00 years ago. Prior to that a
great deal of Utah and Nevada were covered by
Lake Bonneville. What is now Preston was
underwater but current day Downey was dry. Then at
Red Rock Pass
the natural dam broke and sent a wall of water estimated
to be 300 feet high northwest all the way to the Pacific
Ocean. The boulders pictured and the landscape of Marsh
Valley were tumbled and polished by this event.
