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Historical Evolution of the San Pedro River: 8 000 yrs BP to AD 1950 s

Historical Evolution of the San Pedro River: 8 000 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.

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Historical Evolution of the San Pedro River: 8 000 yrs BP to AD 1950 s

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  1. Historical Evolution of the San Pedro River: 8000 yrs BP to AD 1950s Mark A. Gonzalez National Riparian Service Team

  2. Introduction Straw Dog

  3. Project Area San Pedro Riparian National Conservation Area International border to St. David SPRNCA boundary

  4. 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

  5. Holocene Setting Piedmont/Fan Inner Valley Basin Fill Basement Rock

  6. Holocene Setting Inner Valley Pre-entrenchment Landforms Entrenchment Landforms

  7. 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 Qwk Qmc-B Qmc-B Qha Qmc-A Qmc-A Sources: Haynes 1987; Hereford 1993; Waters and Haynes 2001

  8. 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 Qwk Qmc-B Qha Qmc-B Qmc-A Qmc-A 4000 yrs BP 7500 yrs BP 2600 yrs BP Historic 1900 yrs BP Sources: Haynes 1987; Hereford 1993; Waters and Haynes 2001

  9. 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 1450-1850), a globally cooler and wetter period • Channel incision natural process throughout Holocene Qwk Qmc-B Qha Qmc-B Qmc-A Qmc-A

  10. 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 Qwk Qmc-B Qha Qmc-B Qmc-A Qmc-A

  11. 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. Qwk Qmc-B Qha Qmc-B Qmc-A Qmc-A

  12. Holocene Setting: McCool Ranch paleosol ¼ mi S of Boquillas Wash ¼ mi N of Casa de San Pedro 1 mi S Summers Wells Garden Wash Boquillas Wash Qtv Qtv Qtv Qtv Qtv Qmc Qmc Qmc Qmc Qmc

  13. Holocene Setting: McCool Ranch paleosol • Physical Conditions • Little Ice Age (AD 1450-1880) • Low-energy environment • Shallow depth to water table • High organic-matter content (esp. from paleo-cienega) • Water storage and release patterns Qtv Qmc

  14. Pre-entrenchment Conditions • Biological Conditions • Widespread cienega formation • Herbaceous dominated communities

  15. Pre-entrenchment Conditions • Biological Conditions • Sacaton/mesquite bosques on periphery of cienegas

  16. Period of Entrenchment Timing (1880s-1910s +/-, Hereford 1993) • Downstream initiation (1882, Contention area) • Upstream migration (1908 – Hereford Bridge) • Spread into and up tributary drainages

  17. Period of Entrenchment Causes: Ultimately – Big Floods • Changes in climate • Rainfall intensity and frequency • Drought with a few severe storms • Changes in land uses • Mining/Deforestation • Grazing • Combination of 1 and 2 • Tectonic shifts in groundwater levels

  18. 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)

  19. Period of Entrenchment Question remains: Why were floods particularly damaging during 1880s and 1890s?

  20. Period of Entrenchment Causes: Change in land use/changes in basin/upland hydrology High Runoff / Low Infiltration Low Runoff / High Infiltration

  21. 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

  22. 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

  23. Period of Entrenchment Causes • Climate change/pattern? • Intensity, frequency, and amount of rainfall • ENSO activity strong at end of Little Ice Age Precipitation Time

  24. Period of Entrenchment Features: Formation of arroyos San Pedro River Pre-entrenchment channel: Shallow Post-entrenchment channel: Deep

  25. 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

  26. Period of Entrenchment Ecological Consequences: Drop in water table San Pedro River channel Pre-entrenchment water table: Shallow Post-entrenchment water table: Deep

  27. 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.

  28. Period of Entrenchment Ecological Consequences: Loss of water storage West East 4200 ft Inner Valley Pre-entrenchment alluvium V.E. = 100X 4100 ft 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 0 0.5 1 1.5 km Miles 1 0.5

  29. Period of Entrenchment Ecological Consequences: Loss of water storage West 4200 ft East Inner Valley Post-entrenchment alluvium V.E. = 100X 4100 ft 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 0 0.5 1 1.5 km Miles 1 0.5

  30. Channel Evolution • Channel widening: Areal extent calculated from sequential aerial photography for channel 3.2 km N of Hereford bridge Post-entrenchment channels Pre-entrenchment Channel Year 1890-1908 Acres 17 Increase ---- 1937 40 58% 1955 80 50% 1000 m 0 2000 ft (modified from Hereford, 1993)

  31. Channel Evolution Later on the channel narrows and meander rates declined t2 downcutting t2 t3 widening t3 t2 t3 t4 t4 aggradation

  32. 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

  33. 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

  34. 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

  35. Summary Entrenchment period: AD 1890-1908 • Numerous large floods • ENSO strong • Causes? • Climate • Land use • Combination

  36. Summary Post-entrenchment period: AD 1908-1955 • Initial deepening and then widening of channel • High sediment flux • Lost water table and dewatering of alluvium

  37. References Haynes, CV Jr 1987. Curry Draw, Cochise County, Arizona: A late Quaternary stratigraphic record of Pleistocene extenction and paleo-Indian activities, inML 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.

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