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ENVIRONMENTAL AND GEOMORPHIC EFFECTS ON THE LOWER COLORADO RIVER DELTAFROM THE APRIL 4, 2010 EARTHQUAKE, BAJA CALIFORNIA, MEXICO

Steven Nelson1, Francisco Zamora-Arroyo2, and Karl Flessa316101 NE 102nd Avenue #5, Vancouver, WA 98662

2Sonoran Institute, 7650 E. Broadway Blvd., Suite 203, Tucson, AZ 857103Department of Geosciences, University of Arizona, Tucson, AZ 87521

The high-amplitude tides of the northern Gulf of California penetratemore than 40 km up the estuary of the Colorado River. The Delta’smorphology was strongly influenced by these powerful tides even inhistoric times when the river flowed unhindered to the sea. The river hasonly occasionally reached the Gulf since the construction of upstreamdams. The last major river flow through the estuary occurred in 2001. Inthe absence of downstream flow, the tides are transporting sedimentsupstream

The M7.2 El Mayor-Cucapah earthquake in Baja California caused widespread flooding,localized subsidence/lateral spreading, and social and economic disruption in the lowerColorado River Delta. We assessed the earthquake’s effects with sequential satelliteimages, overflights, water-level sensors and ground observations. Most of the flooding in thelower delta was caused by liquefaction, with water coming to the surface through sand boilsand fissures. Some flooding was caused by ruptures of irrigation canals and artificial levees.Liquefaction continued for several days following the earthquake and resulted in ephemeralponds and new drainages. Subsidence resulting from liquefaction has increased the areaaffected by the northern Gulf of California’s high-amplitude tides. Extensive fields of sandboils at the head of the estuary may cause local realignment of the upper estuary/remnantriver channel and may affect the connectivity of wetlands with the Gulf. Local flooding fromliquefaction, subsidence and dike ruptures caused evacuation of some farming communities,disruption of agricultural activity, damage to tourist facilities, and restricted access tocommercial fishing sites. The Ciénega de Santa Clara, a 6,000 ha wetland supported byagricultural drain water from Arizona, was largely unaffected by the earthquake. Water levelsin the Ciénega increased because of diversion of river water away from the delta’s rupturedirrigation canals toward the Ciénega. Water diversion occurred through active managementof the lower river, not because of direct effects of the earthquake.…………………………

ABSTRACT

This Landsat 5 scene of a spring tide shows the pattern of tidal inundation common prior tothe quake. A high tide predicted at 5.17 m occurred about nine hours prior to the satellitepass. The brown area at the upper center is the Ciénega de Santa Clara. The basinsoutheast of the Ciénega is commonly known as the Santa Clara Slough. The flood tideflowed up the Colorado River estuary from the Gulf of California which is just out of view atthe bottom of the image. Upstream flooding was limited by a tidal sand bar blocking theriver channel about 25 km upstream from the Gulf. Flood waters were constrained to theestuary channel by natural levees along the banks. Prior to the earthquake, tides exceeding5.0 meters commonly overflowed a low area on the east bank to inundate the Santa ClaraSlough, as happened in this case.

This Landsat 7 scene was scanned 44 hours after the M7.2 mainshock. The black lines arethe result of a malfunctioning scan line corrector on the spacecraft. The flooding visibleadjacent to and southwest of the estuary indicates the extent of liquefaction. The quakeoccurred during a neap tide period, ruling out the tides as a possible source of the water.New water appearing upstream of the tidal sand bar is the result of both liquefaction andemergency releases of water from damaged canals by irrigation district authorities. TheSanta Clara Slough remains inundated from the previous week’s spring tide. Linearsubsidence features developing on the tidal flats between the estuary and Ciénega de SantaClara reveal rapid post-seismic deformation along southern branches of the El Indiviso Fault(Javier Gonzales, personal communication). No change is visible in the Ciénega itself.

29 March 2010 6 April 2010 30 April 2010 17 June 2010 20 August 2010 7 October 2010

The first post-quake spring tide capable of producing tidal overflows (5.25 m) occurred on 28April. Although this tide was about the same height as that of 29 March, the resulting patternof inundation was significantly different. There was little or no overflow to the east toward theSanta Clara Slough. Instead, the overflows occurred further up the estuary in areas ofsubsidence resulting from the quake. Some of the water found its way into the HorseshoeBend, a meander abandoned by the river in the late 1930s, and into to other areas ofsubsidence southeast of the Bend. North of Horseshoe Bend a new tidal lagoon is visibleindicating subsided ground in the El Indio area. Another tidal lagoon has developed in thedepression of a graben formed along the branches of the Indiviso Fault. The swale createdby the graben provided an avenue for tidal flood waters to reach the large lake to the north,which probably would not have been flooded by a tide of this height before the quake.

The dryer conditions of early summer help to clearly define the limits of tidal inundationproduced by a smaller spring tide. The tide of 13 June apparently did not reach the predictedheight of 5.28 m, but overflows occurred in the same areas observed at the end of April.Once again, no water reached the Santa Clara Slough, which had nearly dried up in theJune heat. The lake west of the Ciénega has also gotten smaller, but received some tidaloverflow via the new graben. Lands along the western edge of the Ciénega de Santa Claraappear much drier than they did in the spring, and the area to the southeast has partiallydried out as well. The dry conditions along the margins of the Ciénega appear to be relatedprimarily to a reduction in inflow during the trial run of the Yuma Desalting Plant.

The highest perigean spring tide of 2010 (predicted to reach 5.84 m) occurred on 10 August.Tidal waters overflowed eastward into the Santa Clara Slough for the first time since thespring tide of 29 March, but the surface area inundated was less than that observed afterthe lower (5.17 m) pre-quake tide. By contrast, tidal inundation to the west was extensive.Flood waters backed up along a 14 km section of the flood control levee south of El Indiviso,infiltrating through earthquake-damaged sections at the north end of the new El Indio tidallagoon. Despite the serious flooding along the levee, two areas south of the levee remaineddry. A postseismic interferogram from the Envisat descending track 313 indicates post-seismic subsidence of up to 15 cm in the area of the flood control levee over the period 13April to 31 August, while the “dry islands” south of the levee appeared stable or showed aslight uplift (Eric Fielding, personal communication).

The height of the 8 September spring tide was similar to that of 10 August. It reached theflood control levee but did not infiltrate it to the extent observed in August. However, itnearly covered the two “dry islands” that remained on the intertidal mudflats in August. TheLandsat scene above was scanned seven hours after a 5.55 m tide on the morning of 7October. The two “dry islands” appear much larger than in August, though still wet afterSeptember’s near-complete inundation. This tide reached the flood control levee in the ElIndio area. Subsidence and inundation of the intertidal zone between the levee and theestuary channel during spring tide events blocks vehicle access to commercial fish campswithin the estuary. The September and October tide events introduced more water into theSanta Clara Slough, though it did not reach pre-quake levels. Wetter conditions around theCiénega de Santa Clara reflect increased bypass drain inflow in September.

tidal sand barPost-seismic deformation

Horseshoe Bend

El Indio Lagoon

levee infiltration

“Dry Islands”

The lower Colorado River Delta is at the head of the Gulf of California near the southern endof the geologically active Salton Trough. Underlying the Gulf and southern half of the Troughis an oblique rift system with short spreading segments connected by long transform faults.The M7.2 Sierra El Mayor-Cucapah Earthquake ruptured along several of these northwest-southeast oriented right-lateral strike-slip transform faults, including the newly-discoveredIndiviso Fault zone which extends from the area of the epicenter southeast through theMexicali Valley to the lower Delta (King 2010, NEIC 2010).

The Colorado River enters the Salton Trough near Yuma, Arizona before flowing through theMexicali Valley to the Gulf. The average elevation of the lower Delta is 3 meters abovemean sea level. The geologically young sediments in combination with a naturally shallowground water table make the Delta highly susceptible to liquefaction during strong groundmotions.

GEOLOGIC BACKGROUND

Lower Colorado River DeltaStudy AreaThe area encompasses theintertidal portion of the Deltasouth of the Mexicali Valleyagricultural zone, including theColorado River Estuary and theCiénega de Santa Clara. Theepicenter was 30 km NE of thestudy area near the base of theSierra Cucapa. The IndivisoFault zone (red line) and othertransverse faults (gold lines)ruptured during the quake.

CHANGING PATTERNS OF TIDAL INUNDATION AFTER THE EARTHQUAKE Predicted tide heights are for Puerto Peńasco, SON (CICESE).

The mainshock of the M7.2 Sierra El Mayor-Cucapah Earthquakeoccurred on Easter Sunday, April 4, 2010 at 3:40 PM PDT. The rupturewas initially slow and involved normal faulting. Following a pause ofabout 6 seconds the main slip release propagated rapidly to thenorthwest with rupture occurring along the Pescadores and BorregoFaults in the Sierra Cucapa Mountains. Simultaneously a less rapidpropagation progressed southeastward along the newly named IndivisoFault toward the Delta (Brandenberg et. al. 2010).

A relatively impermeablelayer of fine surfacesediments overlying a highwater table createsconditions ideal for theformation of sand boils(Yang and Elgamal, 2001).Countless sand boils andsand volcanoes appeared inthe study area along fissuresopened by the quake.

Fissures opened in this fieldas a result of lateralspreading along the marginsof abandoned rivermeanders. Sand boil ejectaand subsidence along theold meanders destroyed thegrade of this field, makingirrigation impossible.

Liquefaction ground failurealso destroyed or damagedmany canals and otherelements of the region’sirrigation infrastructure.

Photo by J. R. Gingery, used bypermission.

Campo Mosqueda is apopular tourist destinationon the Rio Hardy, a tributaryof the Colorado Riverlocated a few miles north ofthe study area. Facilities atthis and other nearbyrecreation sites were heavilydamaged by ground motion,sand boil eruptions andsubsequent flooding.

The quake caused heavydamage and flooding inEjido Luis Encinas Johnson,a farming village of 65families located near theCiénega de Santa Clara.The residents evacuated tohigher ground where theylived in tents supplied by thegovernment. Most have nowreturned to the Ejido torebuild their homes.

The southern edge of theagricultural zone isprotected from river floodsand tidal intrusion by asystem of levees, many ofwhich were damaged bylateral deformation andsubsidence. Sea waterintrusion has occurred (see20 August Landsat below).

Sand volcanoes larger than3 meters in diameter arecommon on the intertidalmudflats south of the levees.

Liquefaction occurred onapproximately 31,000 ha ofthe intertidal mudflats southof the levees. This aerialview of the Horseshoe Bendarea was taken from analtitude of 600 meters. Sandvolcanoes ejected largevolumes of mineralizedwater onto the surface.

A network of fissures withassociated sand boileruptions covers theintertidal plain southeast ofHorseshoe Bend. Pre-quake vehicle tracks lendscale.

REFERENCESBrandenberg, Scott J., John Fletcher, James R. Gingery, Kenneth W. Hudnut, Tim McCrink, Jorge F. Meneses, Diane Murbach,

Tomas Rockwell, Jonathan P. Stewart, and John Tinsley. 2010. Preliminary Report on Seismological and GeotechnicalEngineering Aspects of the April 4 2010 Mw 7.2 El Mayor-Cucapah (Mexico) Earthquake.http://www.geerassociation.org/Post_EQ_Reports.html

King, Nancy. 2010. Report of USGS Activities, El Mayor-Cucapah Earthquake of Sunday, April 4, 2010, Nancy King, ScientificResponse Coordinator, Tuesday May 25, 2010. http://tec.earth.sinica.edu.tw/tec/images/El_Mayor_Cucapah_earthquake_Report.pdf

NEIC 2010. USGS National Earthquake Information Center USGS/NEIC (PDE) database search. http://earthquake.usgs.gov/earthquakes/eqarchives/epic/epic_rect.php

Yang, Zhauhui, and Ahmed Elgamal. 2001. Sand Boil Mechanisms and Effects on Liquefaction-Induced Ground Deformations.Proceedings of the 15th Intl. Conference on Soil Mechanics and Geotechnical Engineering, Istanbul, Turkey, 345-359.

ACKNOWLEDGEMENTSSpecial thanks go to Leigh Fall for producing the study area map and to JavierGonzalez for offering valuable insights in the interpretation of the Landsatimages. Steven Nelson thanks the Research Coordination Network: ColoradoRiver Delta (NSF Grant 0443481) for support.

COLORADO RIVER ESTUARY: IMPACTS TO RESTORATION

Tidal sand bar at spring tide, pre-quake. Same view at low tide, pre-quake. Same view post-quake: channel obliterated.

Sand Bar

To Gulf21 km

PotentialPilot ChannelTo Enhance Connectivity

Old ChannelObliterated

New TidalLagoon In Subsided

Area

Headcuts

CompromisedLevee

ASTER 18 May 2008 ASTER 12 August 2010

Aerial view to east from obliterated sand bar channel to new tidal lagoon. Headcut

upstream and depositing them in a sand bar 25 km above the river’smouth. The bar prevents the mixing of sea water and fresh water that isessential to the productivity of the estuary. The Sonoran Institute and itspartners have embarked on an ambitious plan to restore the Delta’sestuary by securing increased fresh water flow from irrigation returns andtreated wastewater effluents. They are also evaluating the feasibility ofmaintaining an open channel through the sand bar.

Feasibility studies for a sand barproject were initiated in 2008.Topographic surveys indicatedthat tidal mixing could besignificantly increased byinstallation and maintenance ofa relatively short (0.8 km) pilotchannel across the crest of thebar. Project design wasunderway at the time of theearthquake.

The earthquake caused heavyliquefaction in the sand bararea. Sand boils and sandvolcanoes erupted within theriver channel and on adjacentlands. The shallow remnantchannel became clogged withsand boil ejecta. Post–seismicsubsidence disrupted thesurface gradient, renderingprevious topographic surveysobsolete.

The loss of channel integrity willlikely reduce tidal/fresh waterconnectivity within the estuaryand significantly complicaterestoration efforts.

Lands subsided in the El Indioarea 3 km to the east of thesand bar. Ejected water soonpooled in this area (see 6 AprilLandsat scene), and a new tidallagoon appeared on the springtide (see 30 April scene). Post-quake MODIS scenes show theEl Indio area flooded on mosttides exceeding a predictedheight of 4.8 m.

Sea water now floods andrecedes from the El Indio lagoonfor 6-8 days each month.Because the bed of the lagoonis situated several meters abovethe bed of the estuary channel,a new drainage network is beingheadcut toward the north. Thehead of the longest of thesenew drainage channels is nowmore than 4 km north of itsobserved position in early June.

Although the old channel belowthe sand bar floods during thehighest spring tides, it shows noevidence of post-quake channelscour. .

CIÉNEGA DE SANTA CLARA

Earthquake

YDP Startup

water level sensor #1

The Ciénega de Santa Clara complex of dense cattail marsh, open ponds and mudflats islocated directly on the Cerro Prieto Fault and just east of the southern end of the newlydiscovered Indiviso Fault. The wetland was created in 1977 when the U.S. began sendingbrackish groundwater from the Wellton-Mohawk Irrigation and Drainage District insouthern Arizona to this area via a 100-km concrete-lined bypass canal. This watercreated the largest marsh in the entire Sonoran Desert, a wetland of major ecologicalsignificance. In addition to providing habitat for several endangered species and astopover for waterfowl migrating along the Pacific flyway, the Ciénega yields tourismrevenue for the adjacent community of Ejido Johnson.

Water elevations within the Ciénega were stable in the weeks following the quake. Waterdeliveries actually increased for several weeks as authorities diverted water into thewetland while performing inspections and repairs to damaged irrigation infrastructure.Subsequent reductions in water level over the summer can be attributed to the reductionin inflow because of the trial run of the Yuma Desalting Plant (YDP) and do not appear tobe related to earthquake effects.

A topographic and bathymetric resurvey is in progress to identify any quake-relatedchanges from 2009 baseline data. The only change apparent to date is the closure of anarrow channel leading to one of the water quality sensors.

CONCLUSIONS

.ASTER Scene 8 May 2010

Total inflow vs. water elevation at sensor #1 (interior Ciénega).

Colorado River estuary

Santa Clara Slough

Ciénega de Santa Clara

LiquefactionZone

East Lagoon(in graben)

Fish camps no longer accessible by road from the north.

1. Liquefaction occurred on approximately 31,000 ha of the intertidal mudflats south ofthe protective levees.

2. Post-liquefaction and postseismic subsidence has changed the pattern of tidalinundation of the intertidal zone by creating several new tidal lagoons/basins.

3. Liquefaction at the tidal sand bar blocking the estuary channel has reduced channelintegrity over a linear distance of about 6 km, which may further reduce tidal/freshwater connectivity and complicate efforts at restoration. Loss of channel integrity incombination with subsidence in the El Indio area to the east may result in realignmentof the channel upon resumption of river flow.

4. The Ciénega de Santa Clara, a 6,000 ha wetland supported by agricultural drain waterfrom Arizona, was largely unaffected by the earthquake.

LIQUEFACTION/GROUND FAILURE