ROBERT H. WEBB and DIANE E. BOYER
U.S. Geological Survey
1675 W. Anklam Road
Tucson, AZ 85745
LYNN ORCHARD and VICTOR R. BAKER
Department of Hydrology and Water Resources
University of Arizona
Tucson, AZ 85712
The
San Juan River is an important source of water for agriculture and domestic
use, is home to several endangered fish species, is a valued whitewater recreation
area, and supports valued riparian habitat. This major tributary of the Colorado
River drains 60,000 km2 (23,000 m²) of the Four Corners Region upstream
from Mexican Hat, Utah, and has its headwaters in the San Juan Mountains of
southwestern Colorado. The river cuts a course westward through northern New
Mexico and southern Utah until it enters Lake Powell reservoir. The river was
unregulated before June 28,1962, when Navajo Dam was completed; the dam's floodcontrol
operations reduce peak discharges downstream. Before flow regulation, a very
large flood occurred on the San Juan River on October 6, 1911, causing severe
damage; however, the discharge for this flood has only been roughly estimated.
Photographs of the river corridor from the 19th and early 20th centuries show
a wide floodplain with sparse riparian vegetation, and repeat photographs show
a large increase in both native and non-native vegetation. The purpose of this
poster is to evaluate whether the 1911 flood, or other floods in the late 19th
and early 20th century, explain the lack of riparian vegetation along this river,
and whether flood control by Navajo Reservoir has resulted in an influx of riparian
vegetation and channel narrowing.
The
gaging station of the San Juan River near Bluff, Utah (the gaging station actually
is upstream from the bridge at Mexican Hat), has recorded floods from 1914 to
1916 and 1927 to the present. Historically, the largest floods on the San Juan
River occurred between late summer and early fall (August-October). The largest
flood in the gaging record had a discharge of 1,980 m³/s (70,000 ft³/s)
on September 10, 1927; the stage of this flood was about 10 m (31 ft). After
1962 and through 2001, the largest flood on the San Juan River was 1,090 m3/s
(38,500 ft³/s) caused by Tropical Storm Norma on September 6, 1970 (Roeske
et al., 1978). Most of the runoff generated by this storm was downstream from
Navajo Reservoir.
Several floods before the start of the gaging record in 1914 were larger and more damaging than the 1927 flood. Published accounts document two extremely large floods in 1909 and 1911. The 1909 flood occurred in early September and was attributed to heavy and continuous rains in the San Juan's headwaters. Long-time residents of nearby Farmington, New Mexico, stated that the flood was the largest since the settlement of the area in 1880 (Freeman, 1909). A larger flood peaked on October 6, 1911, after a week of heavy rains (Brandenberg, 1911). This flood destroyed most of the bridges on the mainstem San Juan River, including the first Goodridge bridge at Mexican Hat. That bridge was 12 m (39 ft) above the river. LaRue (1925) reported that this flood had a discharge of 4,250 m³/s (150,000 ft³/s), but provided no supporting documentation.
Paleoflood
hydrology involves the reconstruction of the dates and discharges of past floods,
including historical events not recorded at gaging stations. The San Juan Canyon
upstream from Mexican Hat contains an extraordinary amount of slackwater-deposit,
paleostage indicator (SWD-PSI) evidence that allows estimation of the discharges
of past floods (Orchard, 2001). As the river
flows west between Bluff and Mexican Hat, Utah, it cuts perpendicularly through
a series of uplifted monoclines and anticlines in Paleozoic rocks and becomes
entrenched deep in the limestones of the Honaker Trail and Paradox Formations.
The resistant limestones form a narrow, deep canyon with numerous bedrock overhangs
that preserve sediment deposited by large floods. A nearly continuous series
of slackwater deposits and driftwood lines deposited 8-14 m (26-45 ft) above
the channel are present within the 8 km (5 mi) reach (river mile 13 to 18).
Stratigraphic sequences were described at 11 sites, and paleostage indicators
such as driftwood lines have been described at several other sites (Orchard,
2001). Each stratigraphic section contains evidence of 3-5 floods, but as
many as 10 flood deposits are present in some sections. The discovery of humanmade
structures and artifacts buried the highest under flood deposits suggests that
these floods are all historic. The presence of non-native plant materials in
the sediments further underscores that all flood deposits are historic (Orchard,
2001).
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| Aerial photographs of the study reach, including the location of the surveyed cross sections (yellow bars), Eight-Foot Rapid, and the location of prominent slackwater and driftwood deposits (white text). Flow is from right to left. |
Throughout the study reach, well-preserved driftwood lines are sometimes continuous for over 1 km (0.6 mi). These driftwood lines contain historic artifacts, such as sawn wood, metal cans, and nails. Some of the nails have round heads, suggesting they were manufactured after 1900. These high driftwood lines greatly exceed the expected elevations of any measured discharge recorded in the gaging record, and we attribute them to the October 1911 flood. The continuous driftwood lines provide a record of the water-surface slope during this large flood.
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| Slackwater deposits in this reach of the San Juan river are typically found under overhangs created by bedrock outcrops. Driftwood is found at higher elevations than slackwater deposits and typically lies on rocky talus slopes exposed to weathering. |
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| In this view, typical exposure of slackwater sediments is preserved underneath a bedrock overhang. This particular deposit is at river mile 15.1 on river right and contains 6 flood units. |
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