THREE NOVELLAS: PAST, PRESENT
AND FUTURE CONDITIONS OF THE
SAN PEDRO RIPARIAN NATIONAL
CONSERVATION AREA
Mark A. Gonzalez
National Riparian Service Team
OUTLINE
Past, Present and Future Conditions
of the San Pedro Riparian System
Novella 1: Once Upon a Time – Holocene history of
the SPRNCA
Novella 2: Flower Power – Everything you needed to
know about plants but were afraid to ask
Novella 3: Making a Better Sponge – Processes that
store and release water along the San Pedro River
INTRODUCTION
Project Area
San Pedro Riparian
National Conservation
Area
International border to
St. David
SPRNCA boundary
(from Hereford 1993)
NOVELLA I: ONCE
UPON A TIME –
HOLOCENE HISTORY OF
THE SPRNCA
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)
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: 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
PERIOD OF ENTRENCHMENT
Timing (1880s-1910s +/-, Hereford 1993)
Downstream initiation (1882, Contention area)
Upstream migration (1908 – Hereford Bridge)
Spread into and up tributary drainages
San Pedro River
Pre-entrenchment channel: Shallow
Post-entrenchment channel: Deep
CHANNEL EVOLUTIONARY SEQUENCE
t 1 Stable channel
t 2 Downcutting
t 3 Widening
t 4 Aggradation
t2
t1 stable
t3 widening
t1
t2 downcutting
t3
t4 aggradation
SUMMARY
 Stream valleys evolved naturally throughout the Holocene
Period in response to climatic fluctuations.
 Aggradation (valley filling) coincident with wet/cool periods
and high water tables
 Channel incision and sediment removal coincident with
dry/warm periods
Qmc-B
Qmc-A
Qmc-B Qha
Qwk
Qmc-A
Sources: Haynes 1987; Hereford 1993; Waters and Haynes 2001
SUMMARY
Channel Evolutionary Sequence: SPR in ‘infancy’
 t1
 t2
Stable channel
Downcutting
 t 3 Widening
 t 4 Aggradation
t2
t1 stable
t3 widening
t1
t2 downcutting
t3
t4 aggradation
NOVELLA II:
FLOWER POWER -EVERYTHING YOU EVER
WANTED TO KNOW ABOUT
PLANTS BUT WERE AFRAID TO
ASK
PAST –PLANT COMMUNITIES
Channel +
Terrace
Floodplain/Cienega
Perennial Flow
• Obligate wetland
plants: Hardstem
and threesquare
bulrush; flatsedge;
cattail,
PAST –PLANT COMMUNITIES
Terrace
Channel
Floodplain/Cienega
Perennial (water-table
< 20 ft depth)
• FACW plants:
Sacaton;
bunchgrass/shrub;
mesquite (+/dependent on fire
regime)
PRESENT – PLANT COMMUNITIES
Holocene Terraces
Terrace
Perennial Reaches
Obligate, and facultative
wetland species:
hardstem and
threesquare bulrush;
cattail; spikerush;
horsetail, and seepwillow
Channel
Floodplain
PRESENT – PLANT COMMUNITIES
Holocene Terraces
Terrace
Channel
Floodplain
Intermittent Reaches
Johnsongrass,
Bermudagrass,
seepwillow, and minor
occurrences of hydric
herbaceous species
PRESENT – PLANT COMMUNITIES
Holocene Terraces
Channel
Terrace
Perennial Reaches
Fremont cottonwood /
Goodding’s willow;
Baccharis,
with lesser amounts of
netleaf hackberry,
mulberry, grama grasses
Floodplain
PRESENT – PLANT COMMUNITIES
Holocene Terraces
Channel
Terrace
Floodplain
Intermittent Reaches
Fremont cottonwood /
Goodding’s willow; xeric
woody shrub (AZ ash
and walnut, hackberry)
with tamarisk/; baccharis
and herbaceous
understory
PRESENT – PLANT COMMUNITIES
Holocene Terraces
Terrace
Perennial Reaches
Sacaton – Mesquite continuum
Sacaton dominated with
frequent fire
Mesquite dominated with fire
suppression
Channel
Floodplain
PRESENT – PLANT COMMUNITIES
Holocene Terraces
Terrace
Channel
Floodplain
Intermittent Reaches
Increase in woody shrubs
and decrease in
herbaceous plants
Facultative plants more
common than hydric
plants
SUMMARY
Holocene Terraces
Terrace
Channel
Floodplain
Plants in and near the riparian zone ‘ stratify’
themselves by:
 Depth to water
 Permanence / seasonality of water (i.e., Perennial
vs. Intermittent flow)
NOVELLA III:
MAKING A BETTER
SPONGE – STORING AND
RELEASING WATER
WATER STORAGE: BANKS
I Standard Form
II Ponded Form
III Tributary Fan Form
IV Adjustment Form
(I)
(IV)
(I)
(III)
(II)
WATER STORAGE: BANKS
When stage (water level) is high in the channel, water is forced
(pushed by hydrostatic pressure) into the banks and stored in
the floodplain alluvium.
When stage falls in the channel, the hydraulic gradient is
reversed and water flows out of the banks and into the
channel to supply baseflow between high-flow events
WATER STORAGE: BANKS
Ponded water moves out of channel and into banks during high stage.
Stored water also moves down valley with the hydraulic gradient.
(I)
(IV)
(I)
(III)
(II)
WATER STORAGE: FLOODPLAIN
Floodplain recharge has greater surface area than
channel-bank recharge alone. This permits more
water to enter alluvial aquifer in short time.
WATER STORAGE: FLOODPLAIN ROUGHNESS
To increase floodplain infiltration,
Floodplain vegetation must decrease water velocity
VEGETATION RESPONSE
Grazing closure in 1988 (< 25 years ago) has
allowed riparian vegetation to establish and grow.
View from Hereford Bridge:
(left) circa mid-1980s (BLM); (right) 10 years later (photos by D. Krueper, BLM)
WATER STORAGE:
TRAPPED SEDIMENT
WATER STORAGE:
TRAPPED SEDIMENT
AND ORGANIC
MATTER
195 lbs H2O
Water
Holding
Capacity
of
Soil
Organic
Matter
Pounds H2O in 100 Pounds of Soil
200
160
140 lbs H2O
120
100 lbs H2O
80
55 lbs
H2O
40
33 lbs
H2O
1
(modified from Carpenter)
2
4
3
Percent Organic Matter
5
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
LOSS IN 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
St David
14 Escalante
13 St David
12 Smrs-Cnt’n
11 Tombstone
10 Depot
9 Fairbank
8 Boquillas
CONTROLS ON
PERENNIAL
FLOW
Reaches 9-10
Predominantly Intermittent
7 Charleston
6 Escapule
Hwy 90
5 Lewis Spr.
4 Cottonwood
3 Hunter
2 HerefordKolbe
Hwy 92
1 Palominas
Reaches 1-8 Predominantly
Perennial
14
13
12
11
10
9
8
7
6
5
4
3
2
USA AZ
MEX SN
1
 Reaches 1-4 Perennial:
 Underlain by restrictive siltclay layers
 Reaches 5-8 Perennial:
 Gaining reaches with water
upwelling on the east side of
the silt-clay restrictive layer
 Reaches 9-14 Intermittent:
 Bedrock controlled or smaller
basin contributions
Elevation, ft above sea level
7500
East
West
6000
4000
2000
3 miles
3 Miles
4000
3000
Elevation (ft amsl)
Palominas
Hereford
Cottonwood
Lewis Spring
Charleston gage
Boquillas
Fairbank
North
South
SUMMARY: CHANGE IN 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
Pre-entrenchment estimated available water storage:
50,000 acre-ft
Post-entrenchment estimated available water storage:
4000 acre-ft
SUMMARY
Bigger sponges store more water--Sponges get
bigger as:
 Vegetation covers riparian area
 Vegetation slows stream velocity and
enhances infiltration
 Vegetation / organic matter is trapped in
sediment
Bottom Line: More vegetation = more water
storage
SUMMARY
Bigger sponges store more water--Sponges get
bigger as:
 Sediment accumulates and aggrades on
floodplain
 Floodplain widens
 Floodplains are inundated
 Tributary mouth fans (and beaver) pond water
Bottom line: More deposition = more water
storage
SUMMARY
Continued improvement in riparian conditions is
dependent upon seasonal timing of streamflow,
adequate runoff volume and sediment load, and
unrestrained floods. Factors that reduce runoff
volume, increase salinity, change runoff seasonality,
or reduce sediment loads are detrimental to the
riparian community (Hereford, 1993).
Protection of both the flow and the sediment regime
of this river are crucial to its survival.