1. Mark A. Gonzalez National Riparian Service Team HISTORICAL EVOLUTION OF THE SAN PEDRO RIVER: 8000 YRS BP TO AD 1950 S

2. INTRODUCTION Straw Dog

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

4.  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 OUTLINE

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) Little Ice Age (AD 1450-1850) paleosol Qwk Qha Qmc-B Qmc-A Qmc-B Teviston alluvium (Qtv) 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) Little Ice Age (AD 1450-1850) paleosol Qwk Qha Qmc-B Qmc-A Qmc-B Teviston alluvium (Qtv) 7500 yrs BP 4000 yrs BP 2600 yrs BP 1900 yrs BP Historic Sources: Haynes 1987; Hereford 1993; Waters and Haynes 2001

9. HOLOCENE SETTING Qwk Qha Qmc-B Qmc-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

10. HOLOCENE SETTING Qwk Qha Qmc-B Qmc-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

11. HOLOCENE SETTING Qwk Qha Qmc-B Qmc-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.

12. HOLOCENE SETTING: M C COOL RANCH PALEOSOL 1 mi S Summers Wells Garden Wash ¼ mi N of Casa de San Pedro Boquillas Wash ¼ mi S of Boquillas Wash Qtv Qmc Qtv Qmc Qtv Qmc Qtv Qmc Qtv Qmc

13.  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 HOLOCENE SETTING: M C COOL RANCH PALEOSOL Qtv Qmc

14.  Biological Conditions  Widespread cienega formation  Herbaceous dominated communities PRE-ENTRENCHMENT CONDITIONS

15.  Biological Conditions  Sacaton/mesquite bosques on periphery of cienegas PRE-ENTRENCHMENT CONDITIONS

16. Timing (1880s-1910s +/-, Hereford 1993)  Downstream initiation (1882, Contention area)  Upstream migration (1908 – Hereford Bridge)  Spread into and up tributary drainages PERIOD OF ENTRENCHMENT

17. 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

18. 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

19. Question remains: Why were floods particularly damaging during 1880s and 1890s? PERIOD OF ENTRENCHMENT

20. Causes: Change in land use/changes in basin/upland hydrology PERIOD OF ENTRENCHMENT Low Runoff / High Infiltration High Runoff / Low Infiltration

21. 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

22. 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

23. Causes  Climate change/pattern?  Intensity, frequency, and amount of rainfall  ENSO activity strong at end of Little Ice Age PERIOD OF ENTRENCHMENT Time Precipitation

24. Features: Formation of arroyos PERIOD OF ENTRENCHMENT Pre-entrenchment channel: Shallow Post-entrenchment channel: Deep San Pedro River

25. 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

26. Ecological Consequences : Drop in water table PERIOD OF ENTRENCHMENT Pre-entrenchment water table: Shallow Post-entrenchment water table: Deep San Pedro River channel

27. 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.

28. Ecological Consequences: Loss of water storage PERIOD OF ENTRENCHMENT km Miles 0 0.5 1.5 1 1 4200 ft 4100 ft Inner Valley Pre-entrenchment alluvium West East V.E. = 100X Available water volume in SPRNCA pre-entrenchment alluvial aquifer (WV a ) composed of silt loam: WV a = (448 acres/mile X 40 miles) X 20 ft thickness X 1.7”/ft ÷ (12”/ft) WV a = 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): WV a = 50,000 acre-ft + (16,000 gallons X 5 X 3’ X 3600 acres) ÷ (325,851 gals./acre-ft) WV a = 50,000 acre-ft + 2650 acre-ft = 52,650 acre-ft

29. PERIOD OF ENTRENCHMENT km Miles 0 0.5 1.5 1 1 4200 ft 4100 ft Inner Valley West East V.E. = 100X Available water volume in SPRNCA for post-entrenchment alluvial aquifer (WV a ) composed of sand: WV a = (128 acres/mile X 40 miles) X 10 ft thickness X 0.9”/ft ÷ (12”/ft) WV a = 3840 acre-feet Post-entrenchment alluvium Ecological Consequences: Loss of water storage

30.  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-entrenchment Channel Post-entrenchment channels Year 1890-1908 Acres 17 Increase ---- 1955 80 50% 0 1000 m 2000 ft 1937 40 58%

31. Later on the channel narrows and meander rates declined CHANNEL EVOLUTION t2t2 t3t3 t 2 downcutting t 3 widening t 4 aggradation t2t2 t3t3 t4t4

32. 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 CHANNEL EVOLUTION

33. 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

34. 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

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

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

37. 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. REFERENCES

40. Post-entrenchment period:AD 1955-Present  Widening of inset channel at much lower rate  Floodplain formation  Channel narrowing  Lower sediment flux  Stabilization of channel and restoration of physical function SUMMARY

41. PERIOD OF ENTRENCHMENT Features:  Formation of large fans at confluences  High sediment flux from tributaries and from channel entrenchment San Pedro River Tributary