WSP04101

• File Number: 8080
• Description: Section D General Interstate Litigation - Rainbow Bridge Monitoring Program
• State: CO
• Basin: Statewide
• Date: 5/21/1964
• Author: DOI-BOR
• Title: Rainbow Bridge Monitoring Program - Reports 4 through 10
• Water Supply Pro - Doc Type: Report/Study

6.

Rock Mechanics - No additional data.

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time of day,.",,<.adings are made also has a def:;;W;jite effect. In no case
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has the var""iiHon of t:,e rock s,urface over tl1e' lO-inch length of tile
strain gage varied more than aeveral 10 thousandths of an inch and
across the cracks rarely more than several thousandths or hundredtils
of an inch. These variations are not discernible by tile naked eye.

These minute changes in the, roel: noted on the gage stations "lhen
projected over the whole mass of the Bridge becomes a fairly sizable
variation. This gives some indication to the processes that helped
form and shape the Bridge and will eventually 'cause its destruction.
It will be difficult to determine the net amount of maximum flexing
in the Bridge because the base points of the triangulation and ele-
vation surveys are on rock that vary similar to the Bridge but to
SOme lesser degree.

Photogeologv

A photo series covering Rainbow Bridge and Bridge Creek Canyon was
taken during each semi-annual field visit. On ~arch '30, 1976 during
Status Report Period 1/6, 28 photos ''''ere taken when Lake Pm"ell '",s
at elevation 3,664.7; on 8-9 and 10-76 during Period #7,28 photos
were taken with the lake at elevation 3,670.0; on January 27, 1976
during I.'eriod 1/8, 28 photos "'ere taken,-1ith the lake at elevation
3,652.2; and on 8-3 and 4-77, 30 photos were taken with the lake at
elevation 3,648.8. Comparison with earlier sets of the photo
series show no physical change in the rock structure. A subtle
change noted in the photos is that the high water marks at elevation
3',674.93 on July 25, 1975, and 3,672.27 on July 5, 1976 have beco:ae
obscure or faded completely except in a few places as the reservoir
recedes during the drought. Streambed sediments are noted as making
expected slight adjustments as the lake recedes and muds from a
flash flood lodged and flushed away. Shifting of cobbles and small
boulders along the visitor pathways appear to be a slight but con-
tinuing change, and some of these rocks are noted in the photos to
reappear on the streambed.

5.

Moisture Rock Samvles - No additional data.
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7.

Geologic Sections - No additional data.

8.

Seismic Monitoring

'On January 28, 1976, the U.S.C.S. was assisted in installing an L.4
Mark I.'roducts vertical seismograph and microwave repeater station
about 1 mile west of Rainbow Bridge on a high mesa promontory. The
unit relays signals to the U.S.G.S. earthquake center at Golden,
Colorado. The one confirmed local occurrence was the rockfall from
the canyon wall about 1/2 mile northwest' of Rainbow Bridge on August
9, 1976. The rockfall caused a spectacular dust cloud but the quan-
tity of rock was relatively small on a regional scale and was a good
test of the seismographs sensitivity.

Ecology Environmental Effect

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As the reservoir recedes during the drought, the re-emerging rock surfaces are

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