Mark A. Gonzalez

National Riparian Service Team

HISTORICAL EVOLUTION OF THE SAN PEDRO RIVER: 8000 YRS BP TO AD 1950S

INTRODUCTION

Straw Dog

Project Area

San Pedro Riparian National Conservation Area

International border to St. David

SPRNCA boundary

Pre-entrenchment River Conditions (Holocene history)

Entrenchment (1890s-1900s)WhenCausesFeaturesEcological/Environmental Consequences

Channel Evolution (1910s -1950s)Channel wideningFloodplain formationChannel narrowing

OUTLINE

HOLOCENE SETTING

Piedmont/Fan

Inner Valley

Basin Fill

Basement Rock

HOLOCENE SETTING

Inner ValleyPre-entrenchment Landforms

EntrenchmentLandforms

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)

Little Ice Age (AD 1450-1850) paleosol

QwkQhaQmc-B

Qmc-AQmc-A

Qmc-B

Teviston alluvium (Qtv)

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)

Little Ice Age (AD 1450-1850) paleosol

QwkQhaQmc-B

Qmc-AQmc-A

Qmc-B

Teviston alluvium (Qtv)

7500 yrs BP4000 yrs BP

2600 yrs BP

1900 yrs BP

Historic

Sources: Haynes 1987; Hereford 1993; Waters and Haynes 2001

HOLOCENE SETTING

QwkQhaQmc-B

Qmc-AQmc-A

Qmc-B

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 1450-

1850), a globally cooler and wetter period• Channel incision natural process throughout Holocene

HOLOCENE SETTING

QwkQhaQmc-B

Qmc-AQmc-A

Qmc-B

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

HOLOCENE SETTING

QwkQhaQmc-B

Qmc-AQmc-A

Qmc-B

Important points:• Aggradation associated with overall wetter

climates• Entrenchment associated with dry

periods/droughts punctuated by isolated or a few powerful runoff events.

HOLOCENE SETTING: MCCOOL RANCH PALEOSOL

1 mi SSummers

WellsGarden Wash

¼ mi N of Casa de

San PedroBoquillas

Wash

¼ mi S of

Boquillas Wash

Qtv

Qmc

Qtv

Qmc

Qtv

Qmc

Qtv

Qmc

Qtv

Qmc

Physical ConditionsLittle Ice Age (AD 1450-1880)Low-energy environmentShallow depth to water tableHigh organic-matter content (esp. from paleo-cienega)

Water storage and release patterns

HOLOCENE SETTING: MCCOOL RANCH PALEOSOL

Qtv

Qmc

Biological ConditionsWidespread cienega formationHerbaceous dominated communities

PRE-ENTRENCHMENT CONDITIONS

Biological ConditionsSacaton/mesquite bosques on periphery of cienegas

PRE-ENTRENCHMENT CONDITIONS

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 1890s1881–Flood destroys dam upstream of Charleston1887–Local newspapers reported damaging floods Jul. thru Sep.1890–Damaging flood in August1891–Floods caused extensive damage to farms and rr. in Aug.1893–Large flood threatened Fairbank and stalled rr. traffic

south of Benson1894–Large flood washed out dam at St. David and damaged

ranches below in August1896—Extensive flood damage reported July thru October1900—Flood-weakened bridges delay trains1901—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

PERIOD OF ENTRENCHMENT

Low Runoff / High Infiltration High Runoff / Low Infiltration

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?

PERIOD OF ENTRENCHMENT

Qtv

Qmc

Causes: Livestock grazing?

“…the malady of overcrowding is with us in an aggravated form….” Southwestern Stockman, 1890

PERIOD OF ENTRENCHMENT

San Pedro River, mid-1980s, BLM

CausesClimate change/pattern?

Intensity, frequency, and amount of rainfallENSO activity strong at end of Little Ice Age

PERIOD OF ENTRENCHMENT

Time

Pre

cipi

tatio

n

Features: Formation of arroyos

PERIOD OF ENTRENCHMENT

Pre-entrenchment channel: Shallow

Post-entrenchment channel: Deep

San Pedro River

Ecological ConsequencesConversion of low-energy cienega to high-energy stream environment

Conversion of fine to coarse sedimentIncrease in overall sediment load

PERIOD OF ENTRENCHMENT

Ecological Consequences: Drop in water table

PERIOD OF ENTRENCHMENT

Pre-entrenchment water table: Shallow

Post-entrenchment water table: Deep

San Pedro River channel

Ecological Consequences: Loss of water storage

PERIOD OF ENTRENCHMENT

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.

Ecological Consequences: Loss of water storage

PERIOD OF ENTRENCHMENT

kmMiles

0 0.5

0.5

1.51

1

4200 ft

4100 ft

Inner ValleyPre-entrenchment alluvium

WestEast

V.E. = 100X

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

kmMiles

0 0.5

0.5

1.51

1

4200 ft

4100 ft

Inner ValleyWest

East

V.E. = 100X

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

Post-entrenchment alluvium

Ecological Consequences: Loss of water storage

Channel widening: Areal extent calculated from sequential aerial photography for channel 3.2 km N of Hereford bridge

CHANNEL EVOLUTION

(modified from Hereford, 1993)

Pre-entrenchmentChannel

Post-entrenchment channels

Year 1890-1908

Acres 17

Increase ----

1955

80

50%

0 1000 m

2000 ft

1937

40

58%

Later on the channel narrows and meander rates declined

CHANNEL EVOLUTION

t2

t3

t2 downcutting

t3 widening

t4 aggradation

t2 t3 t4

Consequences of channel widening and floodplain formationGreater opportunities to dissipate

stream energy during peak flowsSlower water discharge and greater

water subtractionIncreased volume of floodplain aquifer

for water storagePotential to increase base flow

CHANNEL EVOLUTION

Pre-incision conditions: Holocene periodAlternating periods of aggradation (filling) and

degradation (channel entrenchment)Aggradation: cooler/wetter periods; high water

tableEntrenchment: warmer/dryer periods

SUMMARY

Pre-incision conditions: AD 1450-1850Little Ice AgePeriod of cooler/wetter climateHigh water tableLow-energy riparian environmentLow sediment flux / Soil developmentCienegas widespreadAmple supply of water and lots of water storage

SUMMARY

Entrenchment period: AD 1890-1908Numerous large floodsENSO strongCauses?

ClimateLand useCombination

SUMMARY

Post-entrenchment period: AD 1908-1955 Initial deepening and then widening of

channelHigh sediment fluxLost water table and dewatering of

alluvium

SUMMARY

Haynes, CV Jr 1987. Curry Draw, Cochise County, Arizona: A late Quaternary strat igraphic record of Pleistocene extenction and paleo-Indian activi t ies, in ML Hi l l (ed.), Cordi l leran 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 cl imax communit ies 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 cl imate change in the American Southwest. Geology 29:399-402.

REFERENCES