|
<br />1-12 . Chapter 1
<br />
<br />that form steep canyon walls and smooth scoured
<br />shorelines with little talus.
<br />
<br />The Aisles (RM 117.8-125.5) include Stephen Aisle
<br />and Conquistador Aisle which are characterized by
<br />the reappearance of Tapeats Sandstone (RM 120.0-
<br />I 30 0) also found in Lower Marble Canyon.
<br />A \'erage channel width was 70 m (230 ft) and 48%
<br />of the river bed was composed of bedrock and
<br />boulders.
<br />
<br />The river in Middle Granite Gorge (RM 125.5-
<br />139.9) flows through a combination of Precambrian
<br />sedimentary rock and volcanic and metamorphic
<br />rock consisting of amphibolitic schist, limestones,
<br />diabase intrusives, and granitic plutons. These
<br />relatively resistant materials constrict the river to its
<br />narrowest point in Grand Canyon--23 m (76 ft) at
<br />RM 135.0. Average channel width in this reach is
<br />64 m (210ft), and the bed is composed of 68%
<br />bedrock and boulders.
<br />
<br />The river in Muav Gorge (RM 139.9-159.9) flows
<br />through resistant Precambrian vishnu schist and
<br />zoroaster granite, which constrict the channel to the
<br />narrowest average width of any geomorphic reach in
<br />Grand Canyon--55 m (180 ft). The river bed in this
<br />area has the highest percentage of bedrock and
<br />boulders (78%).
<br />
<br />Eight perennial tributaries flow into the Colorado
<br />River in Region IT. These include Clear Creek (RM
<br />84.1), Bright Angel Creek (RM 87.7), Crystal Creek
<br />(RM 98.1), Shinumo Creek (RM 108.6), Tapeats
<br />Creek (RM 133.7), Deer Creek (RM 136.3), Kanab
<br />Creek (RM 143.5), and Havasu Creek (RM 156.7).
<br />These streams typically have low base flows, which
<br />have little effect on mainstem flows and only local
<br />effects on water chemistry. Occasionally floods
<br />from spring snowmelt or summer thunderstorms
<br />produce high tributary flows which have short-term
<br />effects on mainstem water quantity and quality.
<br />
<br />Region IT has 36 major rapids (Hance, Sockdolager,
<br />Grapevine, 83-Mile, Zoroaster, Pipe Springs, Horn
<br />Creek, Salt Creek, Granite Creek, Hermit, Boucher,
<br />Crystal, Tuna Creek, Sapphire, Turquoise, 104-
<br />Mile, Ruby, Serpentine, Bass, Shinumo, 11O-Mile,
<br />Waltenberg, Forster, Fossil, 128-Mile, Specter,
<br />Bedrock, Dubendorff, Tapeats, 135-Mile, Fishtail,
<br />Kanab, Matkatamiba, Upset, Sinyala, and Havasu).
<br />
<br />Final Report
<br />
<br />Reaion III (Below Havasu Creek to
<br />Diamond Creek)
<br />Region ill was 104.8 km (65.1 mi) long from below
<br />Havasu Creek to Diamond Creek (RM 159.9-226.0)
<br />and was divided into two geomorphic reaches--
<br />Lower Canyon and Lower Granite Gorge (Table 1-
<br />3). Lower Canyon (RM 159.9-213.9) had an
<br />average channel width of 94 m (310 ft), a moderate
<br />slope (0.13%), and a bed composition of only 32%
<br />bedrock and boulders. Lower Granite Gorge (RM
<br />213.9-226.0) had an average channel width of 73 m
<br />(240 ft), a moderate slope of 0.16%, and a bed
<br />composed of 58% bedrock and boulders. The river
<br />inL~erCan~fl~sthrooghsOOim~ary
<br />deposits consisting primarily of Bright Angel Shale,
<br />and the shoreline is characterized by talus with
<br />intermittent alluvial fans. Tertiary lava flows
<br />downstream of RM 180.0 shape much of the
<br />shoreline with emergent boulders and cliffs formed
<br />by columnar basalt. The river in Lower Granite
<br />Gorge flows through metamorphic and sedimentary
<br />features similar to those in the lower portion of
<br />Upper Granite Gorge. The geologic formations
<br />consist primarily of granitic and granodioritic rock
<br />of the Zoroaster Granite Complex intermixed with
<br />Tapeats Sandstone.
<br />
<br />This region has 11 major rapids (l64-Mile, Fern
<br />Glen, Gateway, Lava Falls, 185-Mile, Whitmore,
<br />205-Mile, 209-Mile, 217-Mile, Granite Spring, and
<br />224-Mile) formed mostly by alluvial fans. No
<br />significant perennial tributaries exist in Region ill.
<br />
|