Transcript Mark Gonzalez, Riparian Ecologist (Soils)

HISTORICAL EVOLUTION
OF THE SAN PEDRO RIVER:
8000 YRS BP TO AD 1950 S
Mark A. Gonzalez
National Riparian Service Team
INTRODUCTION
Straw Dog
Project Area
San Pedro Riparian
National Conservation
Area
International border to
St. David
SPRNCA boundary
OUTLINE
Pre-entrenchment River Conditions
(Holocene history)
Entrenchment (1890s-1900s)
 When
 Causes
 Features
 Ecological/Environmental Consequences
Channel Evolution (1910s -1950s)
 Channel widening
 Floodplain formation
 Channel narrowing
HOLOCENE SETTING
Piedmont/Fan
Inner Valley
Basin Fill
Basement Rock
HOLOCENE SETTING
Inner Valley
Pre-entrenchment Landforms
Entrenchment
Landforms
HOLOCENE SETTING
Weik Ranch Mbr. 6500-4300 yrs BP (Qwk)
Hargis Ranch Mbr. 3500-2000 yrs BP (Qha)
McCool Ranch Mbr. 2000 BP to AD 1880 (Qmc)
Teviston
alluvium (Qtv)
Qmc-B
Qmc-A
Little Ice Age (AD
1450-1850) paleosol
Qmc-B Qha
Qwk
Qmc-A
Sources: Haynes 1987; Hereford 1993; Waters and Haynes 2001
HOLOCENE SETTING
Weik Ranch Mbr. 6500-4300 yrs BP (Qwk)
Hargis Ranch Mbr. 3500-2000 yrs BP (Qha)
McCool Ranch Mbr. 2000 BP to AD 1880 (Qmc)
Teviston
alluvium (Qtv)
Little Ice Age (AD
1450-1850) paleosol
Qmc-B
Qmc-B Qha
Qmc-A
Qwk
Qmc-A
Historic
1900
yrs BP
2600
yrs BP
4000
yrs BP
7500 yrs BP
Sources: Haynes 1987; Hereford 1993; Waters and Haynes 2001
HOLOCENE SETTING
Important points:
• Wide-scale valley erosion and entrenchment in early Holocene (8000 –
6500 yrs BP)
• Periods of aggradation (valley filling) and degradation (channel incision
and erosion) repeated throughout the Holocene
• McCool Ranch paleosol likely formed during the Little Ice Age (AD 14501850), a globally cooler and wetter period
• Channel incision natural process throughout Holocene
Qmc-B
Qmc-A
Qmc-B Qha
Qmc-A
Qwk
HOLOCENE SETTING
Important points:
• 15,000 to 8000 yrs BP: Cool/wet climate
• Inner valley stable, gradually accumulating sediment
• Desert basin floor covered in woodlands
• Water tables high
• ~8000 yrs BP (start of Altithermal period): Warm/dry climate
• Water tables dropped; channel entrenchment
• Desert basin floors covered in desert scrub
Qmc-B
Qmc-A
Qmc-B Qha
Qmc-A
Qwk
HOLOCENE SETTING
Important points:
• Aggradation associated with overall wetter
climates
• Entrenchment associated with dry
periods/droughts punctuated by isolated or a few
powerful runoff events.
Qmc-B
Qmc-A
Qmc-B Qha
Qmc-A
Qwk
HOLOCENE SETTING: M C COOL
RANCH PALEOSOL
1 mi S
Summers
Wells
Qtv
Garden
Wash
¼ mi N of
Casa de
San Pedro
Boquillas
Wash
¼ mi S
of
Boquillas
Wash
Qtv
Qtv
Qtv
Qtv
Qmc
Qmc
Qmc
Qmc
Qmc
HOLOCENE SETTING: M C COOL
RANCH PALEOSOL
Physical Conditions
Little Ice Age (AD 1450-1880)
Qtv
Low-energy environment
Shallow depth to water table
Qmc
High organic-matter content
(esp. from paleo-cienega)
Water storage and release
patterns
PRE-ENTRENCHMENT CONDITIONS
Biological Conditions
Widespread cienega formation
Herbaceous dominated communities
PRE-ENTRENCHMENT CONDITIONS
Biological Conditions
Sacaton/mesquite bosques on periphery of
cienegas
PERIOD OF ENTRENCHMENT
Timing (1880s-1910s +/-, Hereford 1993)
Downstream initiation (1882, Contention area)
Upstream migration (1908 – Hereford Bridge)
Spread into and up tributary drainages
PERIOD OF ENTRENCHMENT
Causes: Ultimately – Big Floods
1. Changes in climate
• Rainfall intensity and frequency
• Drought with a few severe storms
2. Changes in land uses
• Mining/Deforestation
• Grazing
3. Combination of 1 and 2
4. Tectonic shifts in groundwater levels
PERIOD OF ENTRENCHMENT
Immediate cause: Series of large floods in 1880s and 1890s
 1881–Flood destroys dam upstream of Charleston
 1887–Local newspapers reported damaging floods Jul. thru Sep.
 1890–Damaging flood in August
 1891–Floods caused extensive damage to farms and rr. in Aug.
 1893–Large flood threatened Fairbank and stalled rr. traffic south
of Benson
 1894–Large flood washed out dam at St. David and damaged
ranches below in August
 1896—Extensive flood damage reported July thru October
 1900—Flood-weakened bridges delay trains
 1901—troublesome floods lo the lower San Pedro area in Aug.
 1904/05—Flood damaged structures and shifted the channel
locally
(Source: Hereford 1993)
PERIOD OF ENTRENCHMENT
Question remains: Why were floods
particularly damaging during 1880s
and 1890s?
PERIOD OF ENTRENCHMENT
Causes: Change in land use/changes in
basin/upland hydrology
Low Runoff / High Infiltration
High Runoff / Low Infiltration
PERIOD OF ENTRENCHMENT
Causes: Deforestation/mining?
Rapid development of mining
claims in watershed during the
1870s
Extensive tree cutting in uplands
at this time for mining and fuel
Changes in upland hydrology?
Qtv
Qmc
PERIOD OF ENTRENCHMENT
Causes: Livestock grazing?
“…the malady of overcrowding is with us in an
aggravated form….” Southwestern Stockman, 1890
San Pedro River, mid-1980s, BLM
PERIOD OF ENTRENCHMENT
Causes
Climate change/pattern?
Precipitation
 Intensity, frequency, and amount of rainfall
 ENSO activity strong at end of Little Ice Age
Time
PERIOD OF ENTRENCHMENT
Features: Formation of arroyos
San Pedro River
Pre-entrenchment channel: Shallow
Post-entrenchment channel: Deep
PERIOD OF ENTRENCHMENT
Ecological Consequences
Conversion of low-energy cienega to
high-energy stream environment
Conversion of fine to coarse sediment
Increase in overall sediment load
PERIOD OF ENTRENCHMENT
Ecological Consequences : Drop in water table
San Pedro River channel
Pre-entrenchment water table: Shallow
Post-entrenchment water table: Deep
PERIOD OF ENTRENCHMENT
Ecological Consequences: Loss of water storage
Assume:
• Available water in silt loam = 1.7”/ft; in sand = 0.9”/ft;
• Pre-entrenchment alluvium is 20’ thick; averages silt loam;
• Pre-entrenchment alluvium averages 0.7 miles (3700’) wide;
• Pre-entrenchment paleo-cienega soils average 3’ thick;
• Soil organic matter holds 16,000 gallons for each percent of
organic matter;
• Post-entrenchment alluvium is 10’ thick; averages sand; and
• Post-entrenchment alluvium averages 0.2 miles (1050’) wide.
PERIOD OF ENTRENCHMENT
Ecological Consequences: Loss of water storage
4200 ft
4100 ft
Inner Valley
Pre-entrenchment alluvium
West
East
V.E. = 100X
0
0.5
1
1.5
0.5
km
1 Miles
Available water volume in SPRNCA pre-entrenchment alluvial aquifer (WVa) composed of silt loam:
WVa = (448 acres/mile X 40 miles) X 20 ft thickness X 1.7”/ft ÷ (12”/ft)
WVa = 50,000 acre-feet
Pre-entrenchment alluvial aquifer (with 3’ thick cienega soil on 3600 acres (1/5 of riparian area) and MODEST
5% organic matter):
WVa = 50,000 acre-ft + (16,000 gallons X 5 X 3’ X 3600 acres) ÷ (325,851 gals./acre-ft)
WVa = 50,000 acre-ft + 2650 acre-ft =
52,650 acre-ft
PERIOD OF ENTRENCHMENT
Ecological Consequences: Loss of water storage
4200 ft
Inner Valley
West
East
Post-entrenchment
alluvium
4100 ft
V.E. = 100X
0
0.5
1
0.5
1.5
km
1 Miles
Available water volume in SPRNCA for post-entrenchment alluvial aquifer
(WVa) composed of sand:
WVa = (128 acres/mile X 40 miles) X 10 ft thickness X 0.9”/ft ÷ (12”/ft)
WVa =
3840 acre-feet
CHANNEL EVOLUTION
 Channel widening: Areal extent calculated from sequential
aerial photography for channel 3.2 km N of Hereford bridge
Pre-entrenchment
Channel
Year
1890-1908
Acres
Increase
0
Post-entrenchment
channels
1937
1955
17
40
80
----
58%
50%
1000 m
2000 ft
(modified from Hereford, 1993)
CHANNEL EVOLUTION
Later on the channel
narrows and meander
rates declined
t2 downcutting
t2
t3 widening
t2
t3
t4
t3
t4 aggradation
CHANNEL EVOLUTION
Consequences of channel widening and
floodplain formation
Greater opportunities to dissipate
stream energy during peak flows
Slower water discharge and greater
water subtraction
Increased volume of floodplain aquifer
for water storage
Potential to increase base flow
SUMMARY
Pre-incision conditions: Holocene
period
Alternating periods of aggradation (filling) and
degradation (channel entrenchment)
Aggradation: cooler/wetter periods; high water
table
Entrenchment: warmer/dryer periods
SUMMARY
Pre-incision conditions: AD 1450-1850
Little Ice Age
Period of cooler/wetter climate
High water table
Low-energy riparian environment
Low sediment flux / Soil development
Cienegas widespread
Ample supply of water and lots of water storage
SUMMARY
Entrenchment period: AD 1890-1908
Numerous large floods
ENSO strong
Causes?
Climate
Land use
Combination
SUMMARY
Post-entrenchment period: AD 1908-1955
Initial deepening and then widening of
channel
High sediment flux
Lost water table and dewatering of
alluvium
REFERENCES
Haynes, CV Jr 1987. Curry Draw, Cochise County, Arizona: A late
Quaternary stratigraphic record of Pleistocene extenction and
paleo-Indian activities, in ML Hill (ed.), Cordilleran Section of the
Geological Society of America, Geological Society of America
Centennial Field Guide Vol. 1,pp. 23 -28
Henrickson, DA and Minckley, WL 1984. Cienegas —Vanishing climax
communities of the American Southwest. Desert Plants 6:131 -175.
Hereford, R 1993. Entrenchment and widening of the upper San Pedro
River, Arizona. Geological Society of America Special Paper 282,
46 p.
Waters, MR and Haynes, CV 2001. Late Quaternary arroyo formation
and climate change in the American Southwest. Geology 29:399 402.