|Dams are nothing new to the Grand Canyon.
is clear evidence that in the very recent geological past (2 million years ago to
400,000), lava spilled into the western Grand Canyon on numerous occasions.
These lava flows were sufficiently thick to block the Colorado River and created lakes
that stretched for hundreds of miles upstream.
The presence of 13 major dams have been documented by a number of researchers including
Hamblin (1994). These dams were big...ranging in height from 200 to 2330 feet tall as
compared to present day height of Glen Canyon dam of 597 feet above river
Each of these 13 dams blocked the Colorado River and created a natural lake very much
like the present day Lake Powell. The river/dam/lake cycle repeated itself
many times with the most recent of these lakes forming 400,000 years ago.
The largest lake was known as "Prospect Lake" named for the dam that formed near
present day Prospect Canyon just downstream from the Vulcan's Throne. The lava dam stood
2330 feet high and the resultant lake that formed was bigger than Lake Mead and Lake
Powell combined! Its narrow configuration was very much like the present day Lake Powell
and the shoreline extended from the western Grand Canyon to just beyond Moab, Utah.
Hamblin estimated that the the lake took 22 years to fill with water, 3,000 years to fill
with sediment and may have lasted for as long as 20,000 to 40,000 years before it was
breached by the Colorado River. Other researchers think the life cycle may have been
shorter due the inherent weaknesses often found in lava structures but none-less-ness
these were lake forming features.
The dams and lakes lasted sufficiently long to leave behind many geologic
features in the Grand Canyon. Lava Falls and Vulcan's Forge are two obvious
features that are well known to river runners but one can find other interesting artifact
deposits tucked away in Grand Canyon , Marble Canyon and Glen Canyon.
One of the lakes (Toroweap Lake) left a large gravel, sand and silt terrace deposit
above Lee's Ferry and sediments from this lake also exist today as the main floor of
Havasu Canyon and the high terrace deposits at Elves' Chasm. It may seem
incredible...but the world famous waterfalls of Havasu Creek may actually plunge down a
series of terraces left behind by the rise and fall of a number of these fresh water
Perhaps the most widespread lake deposits can be attributed to Prospect Lake. It
is an interesting fact that the Havasupai Indians found an agricultural opportunity by
farming an ancient lake deposit of fine silt, protected in a protected side canyon in a
remote section of the Grand Canyon. It was here that they settled and founded the
Village of Havasupia which continues to be occupated even today.
More evidence for prehistoric lakes is visible along the present day shoreline of Lake
Powell. Ancient deposits of silt can be found vertically superimposed upon older
layers of horizontal sedimentary rock in some protected areas and
mud flat cracks and other deposits harken back to a distant time period when still
water processes were much more dominant.
It is no small irony that the site chosen for the present day Glen Canyon Dam was
largely determined by a large nearby deposit of gravel just north of the dam site (a delta
deposit in an ancient arm of Prospect Lake?).
Talk about a recycling project! The deltaic deposits from one ancient dam and
lake providing construction material for a new dam and lake.