i

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations 

Report 2004-5296

In cooperation with the Mountains 

Recreation and Conservation Authority

Geological, Hydrological, 

and Biological Issues 

Related to the Proposed 

Development of a Park 

at the Confluence of the 

Los Angeles River and the 

Arroyo Seco, Los Angeles 

County, California
ii

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Geological, Hydrological, and Biological 

Issues Related to the Proposed 

Development of a Park at the Confluence of 

the Los Angeles River and the Arroyo Seco, 

Los Angeles County, California

By Michael Land, Peter C. Trenham, Daniel J. Ponti, Eric G. Reichard, John C. 

Tinsley, Jonathan Warrick, and Robert W. Meyer

In cooperation with the Mountains Recreation and Conservation Authority

Scientific Investigations Report 2004-5296

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2005

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation: Land, Michael, Trenham, P.C., Ponti, D.J., Reichard, E.G., Tinsely, J.C., Warrick, Jonathan, and 

Meyer, R.W., 2005, Geological, hydrological, and biological issues related to the proposed development of a park at 

the confluence of the Los Angeles River and the Arroyo Seco, Los Angeles County, California: U.S. Geological Survey 

Scientific Investigations Report 2004-5296, 49 p.

Michael Land, Peter C. Trenham, Daniel J. Ponti, Eric G. Reichard, John C. 

Tinsley, Jonathan Warrick, and Robert W. Meyer
v

Contents

CONVERSION FACTORS, VERTICAL DATUM, ABBREVIATIONS, AND ACRONYMS ............ vi

Conversion Factors ................................................................................. vi

Abbreviations and Acronyms ..................................................................... vi

Abstract ................................................................................................... 1

Introduction ................................................................................................ 1

Objectives .......................................................................................... 1

Scope ................................................................................................ 4

Description of study area ........................................................................ 4

Design features of Confluence Park ............................................................ 4

Geology ................................................................................................... 6

Geologic overview of the study area ............................................................ 6

Geologic Setting .............................................................................. 6

Geologic Units ................................................................................. 6

Bedrock Units ........................................................................... 9

Unconsolidated Deposits ............................................................... 9

Geologic Structures ........................................................................... 11

Geological Issues ................................................................................. 12

Surface Faulting .............................................................................. 12

Effects .................................................................................... 12

Potential tasks ........................................................................... 13

Earthquake Ground Motions ............................................................... 13

Effects .................................................................................... 13

Potential tasks ........................................................................... 16

Liquefaction .................................................................................... 16

Effects .................................................................................... 16

Potential tasks ........................................................................... 18

Landsliding .................................................................................... 18

Effects .................................................................................... 18

Potential tasks ........................................................................... 18

Reservoir-Induced Seismicity ............................................................... 18

Effects .................................................................................... 18

Potential tasks ........................................................................... 18

Hydrology ................................................................................................ 19

Hydrologic overview of the study area ......................................................... 19

Hydrologic setting ........................................................................... 19

Hydrologic monitoring and modeling ...................................................... 19

 Hydrological Issues ................................................................................. 21

Surface-water hydraulics .................................................................. 21

Effects .................................................................................... 21

Potential tasks ........................................................................ 26

Surface-water quality ........................................................................ 26

Effects .................................................................................... 28
vi

Potential tasks ........................................................................ 29

Ground-water hydraulics ..................................................................... 29

Effects .................................................................................... 29

Potential tasks ........................................................................... 29

Ground-water quality ........................................................................ 30

Effects .................................................................................... 30

Potential tasks ........................................................................ 31

Biology...................................................................................................... 32

Biologic overview of the study area ............................................................ 32

Biological setting .............................................................................. 32

Biological monitoring and modeling ...................................................... 32

Biological Issues .................................................................................... 34

Wildlife Corridors ........................................................................... 34

Effects .................................................................................... 34

Potential tasks ........................................................................ 34

Wildlife Colonization/Reintroduction ...................................................... 34

Effects .................................................................................... 34

Potential tasks ........................................................................ 35

Non-Native and Pest Species ............................................................... 37

Effects .................................................................................... 37

Potential tasks ........................................................................ 37

Ecotoxicology ................................................................................. 38

Effects .................................................................................... 38

Potential tasks ........................................................................ 38

Options for Restoration of Local Habitat and Ecology .................................... 38

Effects .................................................................................... 38

Potential tasks ........................................................................ 40

Summary and Discussion .............................................................................. 40

References Cited .......................................................................................... 45

Figures

1. Map showing the Los Angeles coastal plain and vicinity, showing location of the 

confluence study area (yellow box—area of figure 2) and selected major faults ........... 2

2. Map showing the the Los Angeles River-Arroyo Seco confluence area and 

selected regional features of the study area .......................................................................... 3

3. Map showing the surface geology, geologic units and structures of the study area ......... 7

4. Geological section A−A’ near the Los Angeles River ................................................................ 8

5. Predicted median peak ground acceleration in the confluence area from a 

scenario M7.1 earthquake on the Los Angeles segment of the 

Puente Hills Thrust .................................................................................................................... 14

6. Map showing the predicted median peak ground acceleration in the confluence 

area from a scenario M6.4 earthquake on the Upper Elysian Park Thrust ...................... 15

7. Map showing the liquefaction and landslide potential for the confluence area .............. 17

8. Map showing the location of precipitation stations, surface-water gaging stations, and 

ground-water wells in the study area and vicinity ............................................................... 20
vii

9. Graph showling the historical maximum daily discharge at the Los Angeles 

River above Arroyo Seco (USGS 11097500, LACDPW F57C-R) for May through 

September, 1932-2003 ............................................................................................................... 22

10. Graph showling the historical maximum daily discharge at Arroyo Seco near 

Pasadena (USGS 11098000) for May through September, 1917-2003 ............................... 23

11. Graph showling the number of days with flow exceeding 50 cubic feet per second 

at the Los Angeles River above Arroyo Seco (USGS 11097500, LACDPW F57C-R)

for May through September 1932-2003 .................................................................................. 24

12. Graph showing the relation between river discharge and daily sediment load for 

the Los Angeles 

River (USGS 1103000; see page 34) downstream from the study area ............................. 25

13. Boxplots showing the concentration of selected constituents in surface water 

from the Los Angeles River, 1966−1977(USGS stations 11097500 and 11097490) ............ 27

14. Graphs showing the annual variation in temperature; relation between dissolved 

oxygen and temperature; and annual variation in specific conductance in water 

from the Los Angeles River, 1966−1977 .................................................................................. 28

15. Graph showing the water-level hydrograph for well 2781 (State of California well 

number 001S014WE003S) in the study area .......................................................................... 30

16. Map showing the watershed boundaries, selected stream gages, and USGS 

biological research monitoring sites ...................................................................................... 33

Tables

1. Native vascular plants documented at sites along the Los Angeles River 

(Wallace, 1993) ........................................................................................................................... 36

2. Native species with potential to establish resident populations within the 

proposed Confluence Park ....................................................................................................... 37

3. Summary of potential general tasks for urban-stream restoration projects and 

specific tasks for the proposed development of Confluence Park ................................... 42
viii

CONVERSION FACTORS, VERTICAL DATUM, 

ABBREVIATIONS, AND ACRONYMS 

Conversion Factors

Multiply By To obtain

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year

cubic foot per second (ft3/s) 0.0283 cubic meter per second

foot (ft) 0.3048 meter

foot per second (ft/s) 0.3048 meter per second

inch (in.) 2.54 centimeter

inch per year (in/yr) 25.4 millimeter per year

mile (mi) 1.609 kilometer

square miles 2.590 square kilometers

Temperature is given in degrees Celsius (°C), which can be converted to degrees Fahrenheit 

(°F) by the following equation:

 (°F) = 9/5 (°C) + 32.

Vertical Datum

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 

(NGVD of 1929)—a geodetic datum derived from a general adjustment of the first-order level 

nets of both the United States and Canada, formerly called Sea Level Datum of 1929.

Altitude: As used in this report, altitude refers to distance above or below sea level.

Abbreviations and Acronyms

CGS California Geological Survey

g acceleration of gravity

GIS geographical information system

LACDPW Los Angeles County Department of Public Works

LADWP Los Angeles Department of Water and Power

M magnitude

MRCA Mountains Recreation and Conservation Authority

Ma millions of years

N standard penetrometer blow count

NE northeast

NPDES National Pollutant Discharge Elimination System

NW northwest

SCCWRP Southern California Coastal Water Research Project 

SE southeast

SW southwest

SMMC Santa Monica Mountains Conservancy

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

VOC volatile organic compounds

WRP water reclamation plans

WRDSC Water Replenishment District of Southern California
Abstract

A new park is being considered for the confluence of 

the Los Angeles River and the Arroyo Seco in Los Angeles 

County, California. Components of the park development may 

include creation of a temporary lake on the Los Angeles River, 

removal of channel lining along part of the Arroyo Seco, 

restoration of native plants, creation of walking paths, and 

building of facilities such as a boat ramp and a visitor center. 

This report, prepared in cooperation with the Mountains 

Recreation and Conservation Authority, delineates the 

geological, hydrological, and biological issues that may have 

an impact on the park development or result from development 

at the confluence, and identifies a set of tasks to help address 

these science issues. 

Geologic issues of concern relate to surface faulting, 

earthquake ground motions, liquefaction, landsliding, and 

induced seismicity. Hydrologic issues of concern relate 

to the hydraulics and water quality of both surface water 

and ground water. Biological issues of concern include 

colonization-extinction dynamics, wildlife corridors, 

wildlife reintroduction, non-native species, ecotoxicology, 

and restoration of local habitat and ecology. Potential tasks 

include (1) basic data collection and follow-up monitoring, 

and (2) statistical and probabilistic analyses and simulation 

modeling of the seismic, hydraulic, and ecological processes 

that may have the greatest impact on the park. The science 

issues and associated tasks delineated for the proposed 

confluence park will also have transfer value for river 

restoration in other urban settings.

Introduction

The Mountains Recreation and Conservation Authority 

(MRCA) is considering plans for developing a new park at 

the confluence of the Los Angeles River and the Arroyo Seco 

in Los Angeles County, California (fig. 1, 2). This location 

is referred to as “the confluence” in the remainder of this 

report. The Arroyo Seco flows into the Los Angeles River 

at the southern end of the Glendale Narrows, just before 

the river enters the coastal plain. As in much of its 51-mi 

reach, the river here flows through a concrete-lined channel. 

Approximately 1,000 ft upstream from the confluence, the 

bottom of the Los Angeles River is unlined. The Arroyo Seco 

also is lined with concrete in this region.

The preliminary plans for the park include creation of a 

temporary lake (El Pueblo Lake) on the Los Angeles River 

using several inflatable rubber dams (City of Los Angeles, 

2004). The lake, whose proposed location is shown in figure 

2, would extend from near the upstream end of the lined part 

of the Los Angeles River channel to about 1 mile downstream 

from the Arroyo Seco. The removal of some channel 

lining along the Arroyo Seco also is being considered. The 

implementation of a plan such as that being considered by the 

MRCA, will be affected by a wide array of scientific issues. 

Objectives

The objectives of this study are to identify the most 

important geologic, hydrologic, and biologic issues related 

to the development of an urban park at the confluence of the 

Los Angeles River and Arroyo Seco, and to present a set of 

proposed work tasks to address these issues.

Although an initial concept for a park at the confluence 

has been developed (City of Los Angeles, 2004), the 

exact design has not yet been determined. Therefore, this 

study considers the range of potential hazards that could 

be encountered at the confluence, or impacts that could 

result from development at the confluence that may include 

impounding water with a temporary dam and possible removal 

of channel lining. Although the analyses are directed at a 

specific project proposed for a very small area, a goal of this 

report is to present a suite of science issues that has transfer 

value for river restoration in other urban settings. 

Geological, Hydrological, and Biological Issues Related to 

the Proposed Development of a Park at the Confluence of 

the Los Angeles River and the Arroyo Seco, Los Angeles 

County, California

By Michael Land, Peter C. Trenham, Daniel J. Ponti, Eric G. Reichard, John C. Tinsley, Jonathan Warrick, 

and Robert W. Meyer
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Figure 1. Los Angeles coastal plain and vicinity, showing location of the confluence study area and selected major faults.

2 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California
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Figure 2. The Los Angeles River-Arroyo Seco confluence area and selected regional features of the study area.

Introduction 3
Scope

This report presents an assessment of the existing data 

and previous studies, delineates the geological, hydrological, 

and biological science issues of highest concern for the 

proposed park development, and identifies a suite of potential 

scientific studies and data-collection needs. The scope of 

work considers issues at the confluence, and, when necessary, 

includes some adjacent areas.

The report addresses geological, hydrological, and 

biological issues in separate sections. For each topical area, 

an overview of study area conditions is presented first. Next, 

specific issues that currently affect, or could potentially affect, 

the confluence area are discussed. A list of potential work 

tasks to help address these issues is then given. 

Description of study area

The Los Angeles River and the Arroyo Seco are two of 

the principal rivers that drain the central Transverse Ranges 

and transport sediment across the coastal plain to the Pacific 

Ocean. The confluence of these two rivers occurs along the 

uplifted margin of the Los Angeles basin (fig. 1). This area, 

referred to in this report as the “Glendale Narrows”, is a 

steep-sided valley, approximately 5 mi in length, that separates 

the Elysian Hills and Santa Monica Mountains to the west 

from the Repetto Hills to the east. The Glendale Narrows 

was formed over the last 4 million years (approximately) 

by progressive erosion and downcutting of the Los Angeles 

River channel in response to broad-scale uplift and folding 

of the Santa Monica Mountains and the Elysian/Repetto 

foothills. Because erosion has been the dominant process in 

this area, the unconsolidated alluvial deposits in the Glendale 

Narrows are relatively thin and lie directly on top of a group of 

consolidated marine sandstone, siltstone, and shale.

The Los Angeles River drains 634 mi2 of the San 

Fernando Valley and coastal plain of Los Angeles. The 

Arroyo Seco has a drainage area of 46 mi2 and originates in 

the San Gabriel Mountains. The pathway of the Los Angeles 

River across the coastal plain has shifted in historical times 

(California Department of Water Resources, 1961). Until 

the early 1800s the river emptied into Santa Monica Bay 

(Gumprecht, 2001). The river currently drops about 3,000 

vertical feet over a reach of approximately 51 mi. 

The first settlements of what was to become the City of 

Los Angeles occurred in the vicinity of the confluence. The 

Los Angeles River was the source of water supply through 

both direct diversions and indirect extraction through the use 

of infiltration galleries (Gumprecht, 2001). The river also was 

the center of a rich ecosystem. The Natural History Museum 

of Los Angeles compiled a review of the historical and current 

biological resources of the Los Angeles River (Garrett, 1993). 

A variety of native species now listed as threatened and 

endangered were formerly present, including grizzly bear, 

California condor, southwestern willow flycatcher, southern 

steelhead, unarmored threespine stickleback, and red-legged 

frog. However, with the channelization of the Los Angeles 

River and urbanization of the surrounding lands, these and 

other species have declined or disappeared.

As Los Angeles grew through the late 1800s and into 

the early 20th century, the Los Angeles River continued to 

be at the centroid of urbanization. The river systems and the 

growing city strongly affected one another. In addition to 

being a source of water supply, the river channel also became 

a site for disposal of waste (Gumprecht, 2001). Flooding along 

the river, especially during 1914, 1934, and 1938, resulted 

in extensive loss of life and property. In response to these 

large storm events, numerous flood control projects were 

implemented along the Los Angeles River and its tributaries. 

These projects included reservoir construction, installation of 

catchment basins, and channelization of the river. Intensive 

channelization began in 1938. 

Today, about 48 of the Los Angeles River’s 51 mi are 

lined with concrete (Los Angeles County Department of 

Public Works, 2004, http://la/dpw.org/wmd/watershed/la

(10/14/2004). In addition to their flood control benefits, these 

projects have had impacts on the overall hydrologic regime—

including surface-water/ground-water interaction—and on 

the ecosystem. The upper part of the Glendale Narrows is 

one of the few portions of the Los Angeles River that is not 

lined on the bottom. Lining was not installed here because 

of rising ground water. The Arroyo Seco also is mostly lined 

from its confluence with the Los Angeles River upstream to 

Pasadena. In addition to the direct effects of the river control 

projects, the river system has been affected by other aspects 

of urbanization. The flow regime increasingly has been 

dominated by discharge from water-reclamation facilities and 

from urban discharge. In addition, the water quality of the 

river has been affected by these new sources. The confluence 

area has a long history of industrial activities (for example, 

Taylor Yard and The Cornfields, shown in fig. 2), which have 

been sources of environmental contamination. 

At the beginning of the 21st century, numerous plans 

are being considered for restoring components of the Los 

Angeles River to more “natural” conditions. One plan under 

consideration by the MRCA is the development of a park at 

the confluence. 

Design features of Confluence Park 

The initial concept for a park at the confluence (referred 

to as “Confluence Park”) is to construct a temporary recreational lake on the Los Angeles River. A tentative plan 

delineated by the City of Los Angeles (2004) would include 

one 6-ft- and two 16-ft-high inflatable dams inundating a 

section of the Los Angeles River channel about 200 ft wide 

and about 4,500 ft long. The river in this area has a slope of 

approximately 0.0044. The average depth of the lake would 

be about 11 ft, ranging from about 6 ft at the upstream end 

to 16 ft at the downstream end. The total volume in the lake 

4 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California
South facing view of the urbanized Los Angeles River and Arroyo Seco confluence (photograph by Dale Cox, U.S. Geological Survey)

of Confluence Park will include site cleanup, native plant 

restoration, landscaping, and creation of walking paths, 

community access, restrooms, a visitor center, a bicycle 

station, benches, educational displays and interpretive exhibits 

that tell the story of the expansion of Los Angeles into its 

original suburbs and [its] relationship to transportation, nature 

and culture” (Santa Monica Mountains Conservancy, 2003). 

The planned Confluence Park and El Pueblo Lake would 

be a part of a larger set of restoration projects to improve 

recreational opportunities, enhance ecological communities, 

and foster economic development along the river from the 

headwaters to the bay (Price, 2003; California Resources 

Agency, 2001; Los Angeles County Department of Public 

Works, 2003). 

would be approximately 220 acre-ft. The approximate location 

of the proposed lake (referred to as “El Pueblo Lake”) is 

shown in figure 2. The upper boundary of the lake would 

be just downstream of the beginning of the lined part of the 

Los Angeles River channel. The lake would also extend 

several hundred feet upstream into the Arroyo Seco. The dam 

structures would be inflated during late spring through late 

summer months. 

The proposed lake would be used for boating. According 

to the initial concept, features adjacent to the temporary 

lake could include docks, boat ramps, cafes, and restrooms. 

In addition to creation of the temporary lake, the MRCA 

also is considering removal of some of the channel lining in 

the Arroyo Seco as part of the project. Confluence Park, as 

expressed by the MRCA, “is envisioned as a series of urban 

spaces that combine public open space, an outdoor classroom, 

safe, attractive pedestrian and bike paths, public art, and 

the vision of a restored riparian ecosystem....Development 

Introduction 5
vegetated with deep-rooted coniferous and oak forests during 

glacial episodes than they were during interglacial episodes 

characterized by a drier climate and chaparral-dominated 

vegetation conditions similar to those of today (McFadden, 

1982). The more dense and deeply rooted vegetation and the 

cooler, wetter climates promoted rock and soil weathering and 

resulted in thicker soil mantles in the upland areas. The denser 

vegetation stabilized slopes and thus reduced the influx of 

sediment to the region’s rivers. Elsewhere in North America 

and Eurasia, the growth of continental and mountain glaciers 

caused worldwide sea level to decrease by approximately 300 

feet, causing coastal rivers, such as the Los Angeles River and 

the Arroyo Seco, to grade to a lower base level. The reduced 

sediment supply and lowered base levels during glacial 

periods caused rivers to downcut and erode their banks. As 

the climate warmed up during the transition to an interglacial 

period, the vegetation thinned and sea level rose. Both of these 

conditions led to stream aggradation as weathered hillslope 

material produced during the glacial period was released into 

the streams and as the overloaded rivers deposited sediment 

from a new, steeper gradient. Eventually, as both sediment 

influx and streamflows decreased during the full interglacial, 

aggradation slowed or ceased and the rivers incised and 

reworked deposits as channels migrated across flood plains. 

These repeated cycles of erosion, stream aggradation, and 

sediment reworking in the confluence area have left laterally 

and vertically complex river terrace and flood-plain deposits 

composed of gravelly to silty sediment. The lithologic 

variability of these deposits affects their geotechnical character 

(for example, density, permeability, shear-wave velocity), 

which has implications for engineering activities associated 

with the confluence restoration project.

Geologic Units

A geologic map (fig. 3) and an interpretive geologic 

cross-section along the alignment of the Los Angeles River 

from Fletcher Drive on the north to Cesar Chavez Avenue on 

the south (fig. 4) shows the general distribution and character 

of exposed and near-surface rocks in the confluence area. 

Although additional rock units are shown, the map and 

geologic section were constructed from available sources 

to specifically highlight (1) sequences of Holocene and 

Pleistocene alluvial units that were produced presumably by 

major climatic fluctuations, and (2) the lithologic character 

within each sequence. Sources used for this compilation 

included geologic maps (Lamar, 1970; Yerkes and others, 

1977), soils maps (Nelson and others, 1919), and water well 

and geotechnical borehole logs from U.S. Geological Survey 

(USGS) files.

Geology

Geologic overview of the study area

Geologic Setting

The Los Angeles River–Arroyo Seco confluence area 

lies at the northern margin of the Los Angeles coastal plain, 

a deep, sediment-filled basin between the Transverse and 

Peninsular Ranges in coastal southern California (fig. 1). 

Bounded beneath Mesozoic granitic rocks and the Catalina 

Schist, the basin contains strata as much as 6 mi thick that 

range in age from Late Cretaceous (65 to 99 million years 

ago [Ma]) to the present (Yerkes and others, 1965). Basin 

inception began in the Early to Middle Miocene (less than 

24 Ma), as a result of regional crustal extension related to the 

creation of the San Andreas Fault system (Atwater, 1970). As 

much as 10,000 feet of marine sediment, with interbedded 

volcanic rocks, was deposited in the basin during the Miocene. 

This depositional regime continued until about 4 Ma when a 

transition to the present-day tectonic regime, dominated by 

strike-slip and reverse faults, began to form the current NW-SE 

trending basin geometry (Yerkes and others, 1965). Initiation 

of compressional tectonics in the region significantly deepened 

the basin and uplifted its margins, and caused great thicknesses 

of marine sediment to be deposited in the basin during the 

Pliocene and Early to Middle Pleistocene epochs. Rapid 

growth of the basin-bounding mountain ranges also began 

during the Pliocene. Over the past approximately 3 Ma, uplift 

and erosion of the surrounding mountains eventually overtook 

basin subsidence, and the combination of tectonic deformation 

and basin-filling gradually produced the configuration of the 

present coastline. Sediment deposited in the basin over the 

past several hundred thousand years, particularly near the 

inner margins of the basin, is primarily of fluvial origin. These 

river-laid deposits are generally quite thin, as most sediment 

now being eroded from the Transverse and Peninsular Ranges 

is transported across the coastal plain and deposited offshore 

(California Department of Water Resources, Southern District, 

1961). 

The alluvial sediment deposited by the Los Angeles River 

and the Arroyo Seco in the confluence area is of Quaternary 

age and reflects several cycles of river aggradation and 

erosion. The Quaternary Period, composed of the Pleistocene 

(1.8 to 0.01 Ma) and Holocene (0.01 Ma to present) epochs, is 

notable for extreme changes in global climate that resulted in 

numerous glacial and interglacial episodes. Although the Los 

Angeles region did not experience glaciation directly during 

the Pleistocene, the glacial periods had weather patterns that 

produced cooler and wetter climates in southern California. 

Consequently the upland regions were more densely 

6 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California
Figure 3. Surface geology, geologic units, and structures of the study area.

Geology 7

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Please click on the figure to open an 11”x17” printable version or go to:

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Figure 4. Geological section A−A’ near the Los Angeles River.

8 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California

Please click on the figure to open an 11”x17” printable version or go to:

http://water.usgs.gov/pubs/sir/2004/5296/04-5296_confluence_park/Fig4-Confluence%20Section.pdf
Bedrock Units

The distribution of Mesozoic and Tertiary-age rocks of 

the confluence area as mapped by Lamar (1970) are shown in 

figure 3. These rocks crop out in the Elysian and Repetto Hills 

and are encountered at depths of less than 200 feet beneath 

the Quaternary alluvial sediment in the Glendale Narrows 

and at depths generally less than 50 feet directly beneath the 

proposed location of El Pueblo Lake. These rocks are typically 

consolidated, either weakly or strongly cemented and, as such, 

provide a nearly impermeable “floor” to the unconsolidated 

alluvial deposits that directly underlie the confluence area.

Feliz Granodiorite (pre-Upper Cretaceous, Mc) – This 

plutonic igneous rock is exposed at the southeastern end of 

the Santa Monica Mountains, north of the Hollywood Fault. 

The rock is part of the Peninsular Ranges basement complex. 

Where exposed, this rock is deeply weathered, fairly soft, and 

dull gray to light brown in color (Lamar, 1970).

Topanga Formation (Middle Miocene, Tt) – This 

unit crops out in the extreme northwest and northeast parts 

of the study area (fig. 3) and consists of siltstone, sandstone, 

and siliceous shale (Ttsl), and well-cemented sandstone, 

calcareous sandstone, and pebble conglomerate (Ttss). 

Sedimentary features indicate that the unit was deposited 

in an open marine environment, most likely by turbidity 

currents. The formation is structurally complex: bedding is 

tightly folded and cut by faults. These rocks are distinguished 

from the overlying Puente Formation on the basis of their 

characteristic blocky weathering, brown color, and dark brown 

coatings on weathered surfaces (Lamar, 1970).

Puente Formation (Upper Miocene, Tp) – Interbedded 

sandstone, siltstone, and shale of the Puente Formation crop 

out extensively in the Elysian and Repetto Hills and flank both 

sides of the Glendale Narrows. The formation is subdivided 

in figure 3 on the basis of the relative abundance of siltstone 

(Tpsl), sandstone (Tpss), shale (Tpsh), or diatomaceous shale 

and diatomite (Tpds). Poorly cemented siltstone and finegrained sandstone, along with diatomaceous shale, occur 

primarily in the uppermost part of the formation, with wellcemented sandstone common below. As mapped by Lamar 

(1970) and shown in figure 4, the dominantly sandy Tpss 

underlies the Glendale Narrows and the proposed El Pueblo 

Lake, with Tpsl occurring south of borehole 2773J. However, 

borehole logs that penetrate Tpss in the Glendale Narrows 

region typically describe the Puente Formation as fine-grained. 

This apparent discrepancy could be due in part to the fact 

that the boreholes are largely penetrating shaly interbeds 

within Tpss, or may result from the relatively poor quality 

and generalized nature of most of the available borehole 

logs. Sedimentary features within the Puente indicate that 

deposition of this unit was dominantly by turbidity currents 

and that the lower part of the Puente Formation in the Elysian 

and Repetto Hills probably constitutes a lobe of the large 

Tarzana submarine fan complex identified from exposures on 

the north slope of the Santa Monica Mountains (Lamar, 1970). 

The structure of the Puente Formation reflects both syn- and 

post-depositional uplift and arching of the unit to form the 

Elysian Park anticlinorium. The axis of the anticlinorium 

approximately parallels the NW-SE trend of the Glendale 

Narrows. In general, rocks to the south of the Glendale 

Narrows in the Elysian Hills dip moderately (about 30-50°) 

to the SW whereas those in the Repetto Hills dip moderately 

to steeply (often >50°) to the NE. Locally the structure, 

particularly in the NE, is overprinted by numerous sub-parallel 

small-scale folds and is broken up by faults. 

Fernando Formation (Pliocene, Tf) – Massive, soft 

siltstone (Tfsl) and sandstone (Tfss; not shown) with minor 

conglomerate crop out along the southern margin of the 

Elysian Hills and may directly underlie the alluvium of the 

Los Angeles River flood plain just to the south of the geologic 

section in figure 4. Available lithologic and faunal data from 

the Fernando Formation suggest that the unit was deposited in 

a marine environment in water depths ranging from shallow to 

more than 4,000 feet (Lamar, 1970).

Unconsolidated Deposits

Compilation and analysis of geologic and agricultural 

soils maps, along with borehole log data, provide a means of 

subdividing the unconsolidated alluvial and colluvial deposits 

of the Glendale Narrows region into units that represent 

discrete aggradational sequences that were likely deposited 

as a result of significant climate change. The soils data also 

provide information on the lithologic character of the surface 

deposits of these sequences that can be used to delineate, in a 

generalized sense, the mixed lithologies contained within each 

sequence. In map view, the fluvial sequences are recognized 

as nested river terrace and flood plain deposits whose 

surface soils display more extensive weathering profiles with 

increasing age. Although these units can be readily identified 

in map view on the basis of their position and soils, correlation 

of the units in the subsurface is difficult given the existing data. 

The best characteristic for identifying unit boundaries 

in the subsurface is evidence of an unconformity expressed 

as a buried soil at the top of each sequence. Unfortunately, 

descriptions from available borehole logs for the most part are 

not sufficiently detailed to identify these weathering profiles. 

Occasionally, sediment color is described in borehole logs, and 

in some cases reports of oxidized colors (reds or yellows) may 

be an indicator of a buried paleosol or zone of more intense 

weathering. Even if additional detailed logs were available, 

however, the likelihood is high that many or most of these 

weathering profiles would have been removed by erosion prior 

to deposition of subsequent sequences. 

To aid in correlation, interpretations in figure 4 rely 

on the characteristic that the overall grain size of fluvial 

sequences typically fine upward from a relatively coarse basal 

deposit (generally a gravelly sand in the confluence area) 

to dominantly fine sand or silt at the top of the sequence. 

Sequence boundaries may therefore occur at the boundary 

between major fining-upward sediment packages that can 

be correlated over large distances. Such correlations are 

Geology 9
complicated in the Glendale Narrows area. For the most part, 

older aggradational sequences have been removed by erosion 

during periods of river down-cutting and lateral channel 

migration. Therefore, only remnants of older deposits, with 

limited lateral extent, are likely to remain. Considering the 

complexity of the fluvial depositional system and the limited 

extent and detail of the borehole log descriptions available, the 

subsurface correlation of depositional fluvial sequences in the 

interpretive geologic section should be considered reasonable, 

but somewhat speculative.

Lower to Middle Pleistocene Deposits (Qpt, Qp1) –

Two presumably Lower to Middle Pleistocene depositional 

sequences are distinguished at the ground surface by their 

geomorphology and their soil-weathering profiles. 

Sequence Qpt occurs at the southern margin of the 

Elysian and Repetto Hills as brown to reddish-brown, 

gravelly, poorly sorted alluvial-terrace deposits whose 

original depositional surfaces have been heavily dissected by 

small drainages (Yerkes and others, 1977). Clayey soils of 

the Altamont Series are developed on these deposits and are 

evidence of considerable age. Sequence Qpt is not visible at 

the surface within the Glendale Narrows, but a poorly sorted, 

reddish-brown to yellow, bouldery deposit is encountered at 

depths below 100 ft, north of borehole 3959D. This deposit 

may be correlative to sequence Qpt. Although no direct 

geotechnical data have been obtained, the poor sorting and 

semiconsolidated nature of this unit suggests that it would 

be of high density (standard penetration blow counts (N) 

greater than 30 within finer-grained facies), relatively low 

permeability, and a shear-wave velocity of between 2,000 to 

2,600 ft/s (Fumal and Tinsley, 1985).

Sequence Qp1 is exposed primarily along the eastern 

margin of the Glendale Narrows where it is typically silty 

(Qp1f), suggesting that much of the preserved sequence was 

derived from local, small drainages in the Repetto Hills, rather 

than being deposited by the Los Angeles and/or Arroyo Seco 

Rivers. Soils of the Ramona Series, which have moderate to 

extensive weathering profiles, are typically developed on this 

unit. Within the subsurface (fig. 4), sediment correlated with 

sequence Qp1 occurs only north of the confluence. Along the 

river profile, the sequence is coarser, consisting primarily of 

a gravelly basal unit and overlying sand. As opposed to Qpt, 

this sequence is typically unconsolidated, but still fairly dense 

(N greater than 30). Permeability is likely to be high in the 

gravelly facies and low in the silty ones. Average shear-wave 

velocities for similar deposits elsewhere in the Los Angeles 

region are typically 1,500 to 2,200 ft/s for the coarse-grained 

facies, and about 1,000 to 2,000 ft/s for the finer-grained parts 

of the sequence (Fumal and Tinsley, 1985).

Upper Pleistocene Deposits (Qp2) – Sequence Qp2 is 

exposed primarily to the north of the Glendale Narrows as 

a broad alluvial terrace. Typically sandy (Qp2m), most of 

the unit is mantled by soils of the Hanford Series or, where 

fine-grained (Qp2f), by the Yolo Clay Loam. In both soils, the 

weathering profile is weak to moderate. In the subsurface of 

the Glendale Narrows, the unit is about 50 to 65 feet thick, but 

thins to the south and is apparently absent south of borehole 

2772L. The geotechnical character of this sequence is likely 

similar to that of sequence Qp1.

Lower to Middle or Undifferentiated Holocene 

Deposits (Qh1, Qla) – Sequence Qh1 is the most extensive 

map unit in the lowland areas and occurs as a broad alluvial 

plain that extends from the Glendale Narrows out across 

the coastal plain. Soils developed on this unit belong to the 

Hanford Series where sandy (Qh1m) and to the Chino Series 

where fine grained (Qh1f). The weathering profiles of these 

soils are weakly developed. Sequence Qh1 likely represents a 

major episode of Early to Middle Holocene river aggradation 

in response to the most recent glacial-to-interglacial transition 

that occurred at the end of the Pleistocene. Although laterally 

extensive, it rarely exceeds 50 ft in thickness within the 

Glendale Narrows. Dominantly sandy, the sequence is likely 

highly permeable in most regions, and loose to medium dense 

(N less than 30). Average shear-wave velocities in similar 

materials in the Los Angeles region range from 500 to 850 ft/s 

in the fine-grained facies, to 1,000 to 1,200 ft/s in the coarse 

sand and gravelly zones (Fumal and Tinsley, 1985).

Qla, as mapped by Lamar (1970), is a landslide deposit 

approximately 650 ft wide that occurs on the steep northeast 

flank of the Elysian Hills adjacent to the Los Angeles River 

just to the north of the confluence. The slide consists of Puente 

Formation rubble that likely failed from oversteepening of the 

slope owing to erosion from the Los Angeles River. Movement 

along the Elysian Park Fault of Lamar (1970), which projects 

beneath the slide debris, could also have contributed to slope 

instability by weakening the Puente Formation locally. This 

slide was last active in 1937 when its movement blocked 

Riverside Drive. It is plausible that grading activities along 

Riverside Drive may have contributed to the 1937 slide 

activity (Lamar, 1970).

Upper Holocene Deposits (Qh2) – Sandy (Qh2m) 

deposits that occur adjacent to the active channel of the Los 

Angeles River (predominantly to the west of the channel) 

and within the modern Los Angeles River flood plain reflect 

intermittent aggradational episodes within the Glendale 

Narrows area as a result of storm-related floods within the 

late Holocene. This unit is thin (less than 20 ft), and mantled 

by soils of the Tujunga Series, which exhibit extremely weak 

weathering profiles. Fine-grained Qh2f deposits typically 

occur as thin colluvial aprons of eroded Puente Formation 

at the base of hill slopes. Comparable materials within the 

Los Angeles region are typically very loose (N less than 15), 

permeable except where very fine grained, and typically have 

low shear-wave velocities (less than about 650 ft/s, Fumal and 

Tinsley, 1985).

Historical Deposits (Qhac, af) – Qhac represents 

aggradation and reworking of sediment within the active Los 

Angeles River and Arroyo Seco channels. The boundaries 

of the active channel (fig. 3) extend beyond the current river 

channels to include areas where the rivers have been known 

to migrate during the last century before channelization. 

These deposits typically consist of coarse sand and gravel and 

10 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California
have virtually non-existent weathering profiles. Geotechnical 

properties of Qhac are likely similar to that of Qh2. The 

“deposit” identified as “af” consists of manmade fill, as much 

as 20 ft thick, placed for flood control levee construction and 

as highway and road fill. The specific physical properties of 

these fill materials are presently unknown. 

Geologic Structures

The dominant geologic structure in the confluence area 

is the Elysian Park Anticlinorium, a NW-trending arch that 

has uplifted the Elysian and Repetto Hills. In addition to 

folding of the Upper Miocene Puente Formation and Pliocene 

Fernando Formation, Lamar (1970) notes that the absence of 

Topanga Formation at the crest of the fold indicates an episode 

of pre-Puente deformation as well. The geologic section (fig. 

4) illustrates the arch in the Puente bedrock surface below the 

Quaternary alluvial fill. The bedrock surface rises from north 

to south, from an altitude of approximately 160 ft in the north 

to greater than 260 ft beneath the proposed Confluence Park, 

and then falls slightly to approximately 230 ft at the south 

end of the section. As presently interpreted, the Pleistocene 

alluvial sequences are shown to thin and eventually pinch out 

from north to south such that only the Holocene sequence rests 

atop the Puente in the vicinity of the proposed Confluence 

Park. These relations indicate that the deformation that is 

producing this fold has continued to be active through at least 

the Pleistocene epoch.

Recent evaluations of the geologic structure of the 

Los Angeles Basin following the 1987 Whittier Narrows 

earthquake (Davis and Namson, 1989; Shaw and Suppe, 

1996; Shaw and Shearer, 1999; Shaw and others, 2000) and 

of small-scale folds in the Repetto, Merced, and Puente Hills 

(Oskin and others, 2000) have indicated that the broad fold 

belt that borders the northern and eastern Los Angeles Basin 

is still active and is the surface expression of movement on a 

series of buried (blind) thrust faults. These faults are thought 

to be capable of producing damaging earthquakes, such as 

the 1987 Whittier Narrows and 1994 Northridge events. 

Although there is considerable uncertainty in estimates of 

the geometry and activity of these faults, recent research 

proposes the existence of two stacked blind thrust faults that 

lie directly beneath the Glendale Narrows, including beneath 

the proposed Confluence Park area (Oskin and others, 2000). 

The shallowest of these is designated the Upper Elysian 

Park Thrust. This fault (not to be confused with the surface 

fault trace designated by Lamar (1970) as the Elysian Park 

Fault; see fig. 3) is modeled as a 50° NE-dipping fault buried 

between 2 and 9 miles deep and with an inferred rupture area 

of about 100 mi2. It is thought to be capable of producing a 

maximum M6.4 earthquake (California Geological Survey, 

2002). The approximate surface projection of the tip of this 

fault as modeled by Oskin and others (2000) is shown in figure 

3. Beneath the Upper Elysian Park Thrust within the study 

area is another inferred fault, the Puente Hills Thrust (fig. 1), 

which has been proposed to be the source of the 1987 Whittier 

Narrows earthquake (Shaw and others, 2000). This fault lies 

beneath the entire confluence area and dips to the north more 

shallowly (25°) than does the Upper Elysian Park Thrust. It 

is also considerably larger, having an inferred surface area of 

more than 300 mi2 (fig. 1). The segment of the Puente Hills 

Thrust beneath the confluence area is inferred to be capable 

of producing an earthquake as large as M7.1 (California 

Geological Survey, 2002).

The confluence also lies near the intersection of two 

major high-angle basin-bounding fault systems that do show 

clear expression at the ground surface. The Santa Monica Fault 

system trends E-NE and bounds the southern margin of the 

Santa Monica Mountains (fig. 1). The Whittier Fault system is 

a NW-SE trending zone that runs through the Puente Hills to 

the southeast and then splits into several NW-SE fault strands 

that project northwestward toward the confluence area. Several 

strands of these fault systems are considered to be active and 

capable of producing moderate to large earthquakes, on the 

basis of their geomorphology and fault length. The closest of 

these to the confluence area is the Hollywood Fault, part of the 

Santa Monica Fault system; the fault’s easternmost end can be 

seen in the northwest corner of figure 3. The Hollywood Fault 

is an E-NE striking high-angle (70°N dip) fault that bounds 

the southern margin of the Santa Monica Mountains and has 

a minimum of 6,000 ft of vertical separation, up on the north 

(Lamar, 1970). Left-lateral strike-slip motion also is likely, 

given the fault’s geometry and orientation, but the relative 

amount of left-lateral movement of the fault is unknown. 

Recent evaluations of the Hollywood Fault suggest that it is 

capable of producing a maximum M6.4 earthquake (California 

Geological Survey, 2002). To the east of the confluence area 

and approximately on trend with the Hollywood Fault, but 

separated from it by the Repetto Hills, is the E-NE striking 

high-angle (75°N dip) Raymond Fault (fig. 1). This fault is 

clearly expressed topographically and shows evidence of both 

left-lateral strike-slip as well as reverse motion. The Raymond 

Fault is inferred to be capable of producing a maximum M6.5 

earthquake (California Geological Survey, 2002).

The northern end of the Whittier Fault occurs more than 

10 miles to the SE of the confluence area, within the Puente 

Hills. It is an active, NW-SE striking right-lateral oblique fault 

whose fault parameters are fairly well constrained (Rockwell 

and others, 1990; Gath and others, 1992). It is presumed to be 

capable of producing a maximum M6.8 earthquake (California 

Geological Survey, 2002). Northwest of the Puente Hills, the 

Whittier fault appears to split into several NW-SE trending 

fault strands that project beneath the alluvium of the San 

Gabriel Valley toward the confluence area (Woodford and 

others, 1954; Kundert, 1955). Within the Elysian and Repetto 

Hills area, Lamar (1970) has mapped an 5-mile-wide complex 

zone of NW-striking faults and NE-striking conjugate faults 

in bedrock that he infers may represent the NW continuation 

of the Whittier fault system. One of these faults, the Elysian 

Park Fault (Lamar, 1970, again, not to be confused with the 

Upper Elysian Park Thrust of Oskin, 2000), is exposed in 

Elysian Park along the NE flank of the Elysian Hills. This 

Introduction 11
fault projects beneath the Glendale Narrows immediately to 

the north of the confluence (fig. 3). Lamar cites stratigraphic 

relations within the Elysian Park area that suggest that 

this fault is pre-Pliocene in age, with a maximum vertical 

separation of about 2,000 ft.

There appears to be a fairly abrupt vertical step in the 

surface of the Puente Formation near the projection of the 

Elysian Park Fault beneath the Glendale Narrows (fig. 4). This 

step (down to the north) is consistent with inferred motion of 

the Elysian Park fault and is also associated with an apparent 

pinch-out of sequence Qp1 and abrupt southward thinning 

of sequence Qp2. If movement on the Elysian Park Fault 

produced these relations, this implies that the fault was at 

least nominally active during the Pleistocene, and therefore 

may still be active today. Lamar (1970) also mapped two 

other unnamed NW and NE-trending faults in the Elysian 

Hills that have been projected beneath the Los Angeles 

River by Yerkes and others, (1977). These fault projections 

also appear to be nearly coincident with relative changes in 

bedrock altitude beneath the alluvium, as well as associated 

disruption of the Quaternary strata. One of these faults projects 

across the Los Angeles River directly beneath the proposed 

location of Confluence Park. It should be pointed out that 

current control on the bedrock topography is sufficiently 

coarse, and therefore, the inferred bedrock “steps” may not 

be nearly as abrupt as shown in figure 4. Instead, the bedrock 

steps may not be fault related at all, but merely small-scale 

variations in the bedrock topography most likely associated 

with differential erosion. Likewise, the inferred stratigraphic 

disruptions associated with these possible fault zones also 

are speculative, given the overall poor quality of the available 

borehole logs. Nonetheless, the proximity of the mapped 

faults to these apparent changes in bedrock altitude leads to 

a reasonable hypothesis that the bedrock altitude changes 

may be fault related. The geologic section in figure 4 reflects 

this possibility, as drawn; however, additional information 

is clearly needed to test this interpretation as well as other 

possible interpretations of these structures.

If, as shown in figure 4, the faults inferred in the geologic 

section indeed have moved during the Quaternary, it is likely 

that they represent old zones of weakness in the bedrock that 

are now accommodating localized stresses that result from 

continued growth of the Elysian Park Anticlinorium, and are 

probably not independent seismic sources. These faults might 

therefore represent sympathetic structures that could move 

small amounts in concert with earthquakes on the underlying 

blind thrusts.

Geological Issues 

Future development of Confluence Park ideally will 

incorporate in its design an understanding of the potential 

geologic hazards, including the potential for any such 

development to exacerbate those hazards, thus increasing 

the risk of economic loss to the neighboring community. 

The following section lists possible geologic hazards that 

should be considered for the Confluence Park or similar types 

of projects, their likely impacts with respect to the current 

Confluence Park proposal, and the kinds of studies or tasks 

that might be conducted to better address these issues.

Surface Faulting

Effects

A localized, but potentially severe, impact of earthquakes 

on the urban environment is the potential for surface 

faulting. Surface faulting occurs when movement along a 

fault breaks through to the surface, resulting in permanent 

ground deformation. Most commonly, surface faulting occurs 

suddenly during an earthquake, but it can also occur slowly as 

a result of fault creep. The amount of displacement depends 

on the magnitude of the earthquake or creep event and on 

the location along the fault. For earthquakes in California, 

observed displacements have ranged from a fraction of an 

inch to more than 20 ft. Needless to say, fault displacements 

of more than a few inches can be devastating to certain kinds 

of structures. Fortunately, fault rupture almost always occurs 

along pre-existing faults; therefore, it is relatively easy to 

mitigate this hazard by keeping critical structures off known 

fault zones. In the State of California, the Alquist-Priolo 

Earthquake Fault Zoning Act of 1972 was passed to prevent 

the construction of buildings used for human occupancy on 

the surface trace of active faults. This Act requires the State 

Geologist to define regulatory zones around the traces of faults 

that have evidence for movement within the last 11,000 years 

and to issue maps of these zones, which are then used by local 

agencies to regulate development projects within the zone.

The risk that surface faulting could affect the Confluence 

Park development appears to be very low. An Alquist-Priolo 

zone has been identified along the Raymond Fault several 

miles NE of the confluence (fig. 1), but no zones are currently 

defined near the proposed Confluence Park. Nevertheless, 

given the quality and paucity of available information, the 

potential for surface faulting probably should be investigated 

further. If surface faulting were to occur when the temporary 

lake was present, resulting damage to the concrete channel 

walls or the dams themselves could promote a rapid loss of 

lake water into the ground or overland, potentially causing 

negative environmental impacts and/or damage to downstream 

areas. Rapid movement of water and possible erosion behind 

the channel walls could also exacerbate damage to the 

channel lining and cause localized ground failure or collapse 

of lining structures. Whether the lake was present or not, 

important park facilities, such as the dam control house and 

the inflatable dams might also be significantly damaged. 

As noted previously, there are several mapped fault zones 

that apparently project beneath or near to Confluence Park 

(figs. 3 and 4), and Holocene activity on these structures, 

while unlikely, cannot be entirely ruled out. Even if these 

faults were active, fault displacements would likely be small 

12 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California
owing to their probable relation to folding of the Elysian Park 

Anticlinorium.

Potential tasks

1. Determine whether faults shown in figure 4 beneath the 

confluence area in fact displace the Puente Formation 

and disrupt Quaternary strata. To do this it would be 

necessary to conduct a shallow, high-resolution seismic 

reflection line to image the bedrock/alluvium interface. 

Such an effort would pinpoint the location of likely 

faults for possible further investigation.

2. If surface faulting within the confluence area appears 

possible based on the above investigation, the faults 

could be presumed to be active and their locations then 

accounted for in determining the placement of critical 

facilities at Confluence Park.

3. If design options require that potential faults 

interpreted from the seismic line be further verified, 

or that the recency or amount of fault displacement be 

known, it is possible to conduct trenching or borehole 

investigations to determine the recency and frequency 

of fault activity, and the amount of fault movement 

per slip event. These parameters can then be used 

to estimate the likelihood and magnitude for future 

displacement for use in facilities design.

Earthquake Ground Motions

Effects

By far the most significant hazard from earthquakes is the 

strong ground motions generated. Earthquake ground motions 

impart a lateral load on structures, and may cause structural 

damage or collapse if the structures are not appropriately 

engineered. Although no location in the Los Angeles region 

is safe from the threat of damaging levels of earthquake 

ground motion, the confluence region is at particularly 

high risk for two reasons. First, several seismogenic faults, 

potentially capable of producing earthquakes of M6 or greater, 

are known or are inferred to occur within a 12-mile radius. 

These include the Hollywood, Raymond, Whittier, Puente 

Hills thrust, and Upper Elysian Park Faults. Strong ground 

motions from earthquakes on these faults would be expected 

in the confluence area given the proximity of these seismic 

sources. Second, the confluence region is underlain by a 

relatively thin accumulation of alluvial sediment characterized 

by a low-shear wave velocity, above consolidated rock with 

a significantly higher shear-wave velocity. The shear-wave 

velocity contrast between the bedrock and overlying alluvial 

units would amplify ground motions at frequencies that 

depend on the thickness of the lower velocity material.

Over the last decade or so, significant effort has been 

made by the USGS and by the California Geological Survey 

(CGS) to develop seismic hazard maps that incorporate such 

factors as seismic source location, ground motion attenuation, 

and local site amplification to estimate the shaking hazard 

from earthquakes in California. These new maps are 

probabilistic: they take into consideration uncertainties in 

the size and location of earthquakes and the resulting ground 

motions that can affect a particular site. Seismic hazards 

maps and calculations by the California Geological Survey 

(2002) represent the shaking hazard in terms of a 10-percent 

probability of exceeding a certain ground motion during a 50-

year time period, which is a typical design level for buildings 

in high seismic areas. This value represents an annual 

probability of 1 in 475 of being exceeded each year, and takes 

into account all known nearby sources, their presumed level 

of activity (likely magnitude and rates of recurrence), and 

generalized site amplification effects, along with inherent 

uncertainties. Based on the California Geological Survey 

model (2002), the predicted peak ground acceleration with a 

10-percent probability of exceedence in the confluence region, 

is 0.57 g. This value approaches some of the largest values 

recorded in the northern San Fernando Valley during the 1994 

Northridge earthquake. Similar analyses can also be done for 

anticipated scenario earthquakes. 

Shown in figure 5 is the median (50-percent probability 

of exceedence) predicted peak ground acceleration for a M7.1 

earthquake on the Puente Hills thrust (OpenSHA Model, 

02/02/04). This structure would likely produce the highest 

ground motions of any nearby fault. The figure clearly shows 

the amplification effect resulting from the alluvium within the 

Glendale Narrows. At the confluence, the median peak ground 

acceleration is predicted to be greater than 0.7 g

—considerably higher than the probabilistic value because 

this approach presumes that the Puente Hills earthquake has 

actually occurred. The 10 percent probability of exceedence 

for this earthquake is even higher still (greater than 1.1 g). 

With the exception of the site effects, there is little evidence 

of ground-motion variability because the fault plane lies 

completely beneath the map area and therefore all points on 

the map are approximately the same distance from the seismic 

source. 

Predicted ground motions for a M6.4 earthquake on 

the Upper Elysian Park fault described by Oskin and others 

(2000) are shown in figure 6. The ground motions from this 

earthquake at the confluence are predicted to be less than those 

for the Puente Hills thrust (median peak ground acceleration 

of approximately 0.42 g) because of the lesser earthquake 

magnitude, and because modeled ground motions decrease 

southwest of the fault owing to attenuation.

One unique aspect of the ground motion issue relevant 

to Confluence Park and El Pueblo Lake is the possibility of a 

seiche being created in the lake from an earthquake. A seiche 

is a free or standing wave oscillation of a body of water, 

Introduction 13
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Figure 5. Predicted median peak ground acceleration in the confluence area from a scenario M7.1 earthquake on the Los Angeles 

segment of the Puente Hills Thrust.

14 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California
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Figure 6. Predicted median peak ground acceleration in the confluence area from a scenario M6.4 earthquake on the Upper Elysian 

Park Thrust.

Geology 15
whose period of oscillation is determined by the resonant 

characteristics of the containing basin (Bates and Jackson, 

1980, p. 568). Depending on the frequency content of the 

input ground motions, oscillation of the lake surface might 

prove significant enough to overtop the portable dams. Also, 

wave oscillation would likely impart significant dynamic 

lateral loads to the portable dam structures. If the anchors for 

the dams are not sufficiently strong to withstand these loads, 

damage to the dams could occur and cause an unanticipated 

release of lake water downstream.

Potential tasks

1. Derive design-earthquake ground motions using 

industry-standard techniques, and design buildings 

and engineering works to withstand these anticipated 

ground motions.

2. Determine a shear-wave velocity profile for the 

confluence area to a depth of at least 100 feet to better 

model near-surface site amplification effects. Shearwave measurements could be made in a borehole 

drilled for other purposes (such as a nearby groundwater monitoring well).

3. Model the proposed El Pueblo Lake to determine 

the likelihood of a seiche developing in the structure 

during an earthquake. Evaluate the design of the 

clamps that anchor the portable dams in place to 

determine if they could withstand lateral loads 

imparted by the oscillations.

Liquefaction

Effects

Liquefaction is a secondary effect of earthquake ground 

motion and has historically been a major cause of earthquake 

damage in southern California (California Geological Survey, 

1998). Liquefaction is a process by which water-saturated, 

granular sediment (such as sand) transforms into a liquefied 

state and fails owing to increased pore-water pressures from 

strong ground shaking (Youd, 1973). The geologic and 

hydrologic conditions that favor the occurrence of liquefaction 

are fairly well understood, and susceptible environments 

can be delineated with reasonable precision. As a result of 

the Seismic Hazards Mapping Act of 1990, the California 

Geological Survey has begun producing maps that delineate 

liquefaction hazard zones. Within these regions, subsequent 

site-specific liquefaction analyses are required for certain 

kinds of developments. These liquefaction zones are intended 

to be conservative; they highlight areas of loose, generally 

sandy materials of historical or Holocene age, where ground 

water is thought to be sufficiently shallow (water table within 

40 ft of the land surface), and where potential earthquake 

ground motions are expected to be sufficiently high to produce 

liquefaction (California Geological Survey, 1998). Because 

of inherent variability within the subsurface materials, it is 

improbable that all regions within a liquefaction zone are 

susceptible to failure. The California Geological Survey 

liquefaction hazard map for the confluence area is shown in 

figure 7. Virtually all of the Glendale Narrows, Arroyo Seco, 

and Los Angeles River flood plain areas are included in the 

State’s liquefaction zone.

Insufficient information exists to perform a detailed or 

site-specific liquefaction hazard analysis for the confluence 

area, but a general assessment of the hazard can be made. 

As part of the construction of the geologic section in figure 

4, water-level measurements from borehole records were 

compiled to provide a snapshot of the ground-water conditions 

in the confluence area. These data reflect water levels 

reported when the holes were drilled, and most of them were 

drilled several decades ago. Nevertheless, these data suggest 

a ground-water surface with a gentle southward gradient. 

From the Pasadena Freeway northward, these data show that 

the lowermost portion of Holocene sequence Qh1 is largely 

saturated at depths of approximately 40 ft or less, consistent 

with a fairly high potential for liquefaction. South of the 

Pasadena Freeway, however, and in the area of Confluence 

Park, the ground-water levels lie at or below the base of Qh1. 

Given that the underlying Pleistocene alluvium and Puente 

Formation are not likely to liquefy, the available information 

would suggest that the liquefaction hazard in the vicinity of 

Confluence Park is less than it is farther to the north. However, 

this interpretation needs to be corroborated with more detailed, 

up-to-date information on current water levels, and better 

details about the specific character of the deposits in the 

vicinity of Confluence Park.

One factor that needs to be considered in assessing the 

liquefaction potential of the confluence area is the degree 

to which the development of an intermittent lake may cause 

ground water to rise in areas adjacent to the lake. As present 

plans are for a shallow lake that only floods the lined portion 

of the Los Angeles River, the effect on ground-water levels 

would be minimal. If channel-lining removal is conducted on 

the Arroyo Seco, this may result in a slight rise in groundwater levels. If so, even a modest rise could significantly 

increase the liquefaction hazard near Confluence Park.

If liquefaction were to occur in the confluence area, the 

ground could fail in a number of ways. Common modes of 

ground failure from liquefaction include lateral spreading, 

flow failure, ground oscillation, and loss of bearing strength 

(Youd, 1978a,b). Flow failure is not a likely process in the 

confluence area because of the relatively low slope of land 

surface within the region. The most likely failure mechanism 

in the confluence area is lateral spreading, where gravitational 

forces plus inertial forces from the earthquake can cause 

masses of earth material to move several feet or more toward 

16 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California
Figure 7. Liquefaction and 

landslide potential for the 

confluence area. Map is 

derived from a portion of 

the California Geological 

Survey Seismic Hazard Zone 

Map for the Los Angeles 

Quadrangle (1998). Green 

delineates areas of potential 

liquefaction, as determined 

by historical occurrences of 

liquefaction, or susceptible 

geological, geotechnical, and 

ground-water conditions. 

Light blue delineates areas of 

potential earthquake-induced 

landslides, as determined 

from the prior occurrence 

of landslide movement, or 

local topographic, geological, 

geotechnical, and subsurface conditions that indicate 

a potential for permanent 

ground displacements.

a cut slope or free face. A plausible scenario might be the 

collapse of the banks of the Los Angeles River into its 

channel, causing significant damage to the channel lining as 

well as to any structures near the river bank. If the lake is full 

at the time, the bank failures could displace water from the 

lake and thereby cause overtopping of the dam, an unplanned 

discharge downstream, and local flooding, followed by a 

loss of lake water into the subsurface. Ground oscillations 

and bearing failure under structures could also occur, which 

typically results in localized ground settlement and building 

damage.

Geology 17

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As mentioned previously, one active landslide mass 

exists adjacent to the Los Angeles River channel on the steep 

northeast slope of the Elysian Hills between the Pasadena 

Freeway and Interstate 5. Lamar (1970) speculated that this 

slide formed as a result of oversteepening of the slope caused 

by erosion from the Los Angeles River channel. The Los 

Angeles River channel banks are now concrete-lined, and 

current Confluence Park development plans do not call for 

removal of the lining at this location. However, any future 

restoration plans that might involve channel-lining removal, 

such as in the Arroyo Seco or elsewhere, should take into 

account the possibility that such action may exacerbate a 

slope-failure hazard, especially along the northeast flank of the 

Elysian Hills. There is virtually no hazard from landslides in 

the vicinity of Confluence Park as this location is sufficiently 

removed from the hillslopes. 

Potential tasks

To address the effects of channel-lining removal along 

the Arroyo Seco, slope stability analyses should be conducted 

where the current river channel lies near or adjacent to slopes 

with a gradient of 20 percent or greater.

Reservoir-Induced Seismicity

Effects

Several types of engineering developments are 

suspected, with varying amounts of evidence, to be capable 

of triggering earthquakes on a limited, localized scale 

(Simpson, 1976). The sizes of these triggered earthquakes 

range from moderately sized, damaging earthquakes down 

to micro-seismic emissions, which are only sensed by 

specialized instrumentation. Historically, the development 

of large reservoirs for power generation was the first type of 

engineering activity that significantly affected the seismicity of 

the earth’s crust. The induced seismicity associated with such 

stress modifications was then recognized. The largest event 

of this type yet recorded was a 6.5-magnitude event (Richter 

scale) and this event was associated with the impoundment 

of a very large (deep) reservoir. It is thought that deep new 

reservoirs can trigger earthquakes because of a change in 

stress caused by the mass of the water in the reservoir, or 

more commonly by increased ground-water pore pressure 

that results in a decrease in the effective strength of the rock 

beneath the reservoir. Where reservoir-induced seismicity has 

been observed, the reservoirs involved have been deep to very 

deep (260 to 500 ft, Schwartz and others, 1996). Given that 

proposed creation of El Pueblo Lake will produce a structure 

no deeper than about 16 ft, and the lake will be completely 

lined in concrete, there is virtually no risk that the presence of 

such a shallow lake could trigger earthquakes. 

Potential tasks

None.

Potential tasks

The threat of damage to Confluence Park facilities and 

the engineered channel linings from liquefaction is a plausible 

one, given current knowledge of the subsurface conditions. 

Furthermore, development of a lake could increase the hazard 

locally, potentially putting structures outside the Park at greater 

risk of damage. Unfortunately, existing information is not 

sufficient to determine if liquefaction is indeed likely to occur. 

Potential tasks to develop this information are as follows:

1. Conduct a detailed subsurface investigation of 

the confluence area to evaluate the liquefaction 

susceptibility of the underlying sediment. Such 

an investigation should involve the use of cone 

penetrometer soundings in addition to several 

geotechnical boreholes where samples can be collected 

for lab testing. Piezometers should also be installed 

to measure baseline water levels and to monitor both 

seasonal water-level fluctuations, as well as any water 

level change that may occur as a result of lake filling.

2. Perform liquefaction hazard analyses using the new 

data to assess the potential problem and as input in 

facilities design.

Landsliding

Effects

Landslides have historically been a major cause of 

damage to the built environment and can be triggered by 

storms and earthquakes (California Geological Survey, 1998). 

Landslides have destroyed or damaged homes and other 

structures, blocked transportation corridors, and damaged 

various types of lifelines and other infrastructure. Typically, 

areas most susceptible to landslides are on or adjacent to 

steep slopes and on or adjacent to existing landslide deposits, 

especially if the earth materials in these areas are composed of 

loose colluvial soils, or poorly cemented or highly fractured 

rock. Within the confluence region, geologic and terrain 

conditions favorable to landslides exist on steep slopes within 

the Elysian and Repetto Hills. Earthquake-triggered landslides 

also fall under the scope of the California Seismic Hazards 

Mapping Act of 1990. Figure 7 shows the location of these 

landslide-susceptible zones in the confluence area.

Earthquake-triggered landslides most typically fall under 

the classification of shallow soil slips or rock avalanches; 

rarely is the whole mass of a deep-seated landslide reactivated 

in earthquakes (Keefer, 1984). However, it has been observed 

worldwide in prior earthquakes that the steep scarps and 

toe areas of existing large landslides that formed from prior 

earth movements, are particularly susceptible to earthquakeinduced slope failure (California Geological Survey, 1998); 

therefore, areas having existing landslides are considered to be 

particularly vulnerable to renewed ground failure during future 

earthquakes.

18 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California
Hydrologic monitoring and modeling

Surface-water and ground-water monitoring programs 

and modeling efforts of the study area have been implemented 

by several agencies. These are summarized below.

The location of established stream-gaging stations on 

the Los Angeles River and tributaries in the area is shown in 

figure 8. There are a few active gages near the confluence area. 

The Los Angeles River just above the Arroyo Seco (USGS 

11097500; also LACDPW site F57-CR), and the Arroyo Seco 

below Devils Gate Dam in the Raymond Basin (LACDPW site 

F277-R) are operated by the Los Angeles County Department 

of Public Works and were established in 1931 and 1942, 

respectively. A third site, the Arroyo Seco near Pasadena 

(USGS 11098000) has continuous flow data extending back 

to 1916. The long-term records for these stations provide a 

basis for characterizing the flow regimes under historical and 

recent conditions. A quantitative analysis of the flow regimes 

Hydrology

Hydrologic overview of the study area 

Hydrologic setting

The headwaters of the Los Angeles River originate at the 

western end of the San Fernando Valley and are fed by several 

creeks which drain the surrounding Santa Monica, Santa 

Susana, and San Gabriel Mountains. The river passes through 

the Glendale Narrows, then flows across the coastal plain and 

empties in the Pacific Ocean via San Pedro Bay (fig. 1). The 

Arroyo Seco, a tributary to the Los Angeles River, originates 

in the San Gabriel Mountains, drains part of the Raymond 

Basin, and enters the Los Angeles River at the lower end of the 

Glendale Narrows. The Glendale Narrows links the inland upper 

ground-water basins to the lower coastal ground-water basins. 

Ground water in the Glendale Narrows primarily flows through 

the relatively shallow unconsolidated deposits shown in figure 3

and 4. 

As mentioned in the Introduction, the hydrologic system 

has been greatly affected by urbanization throughout the 

development of the city. Both the Los Angeles River and 

the Arroyo Seco have been channelized for flood-control 

purposes. Upstream reservoirs have altered the natural flow 

regime. Flow throughout most of the year is now increasingly 

dominated by discharge from wastewater reclamation 

facilities, ground-water treatment facilities, and urban runoff. 

As a consequence, the quality of water in the river has been 

affected by these new sources. 

Upstream view near an unlined section of the Los Angeles River. (photograph by Dale Cox, U.S. Geological Survey)

Urbanization also has affected ground-water conditions. 

The source of ground water in the study area is from (1) 

underflow from upper, inland basins, (2) surface water (which 

is often a mixture of winter stormflow, treated wastewater, 

urban runoff, and treated ground water), and (3) direct 

infiltration of precipitation. 

The unlined part of the Los Angeles River channel (see 

fig. 2) has historically been an area of natural ground-water 

discharge. Ground-water pumpage within the Glendale 

Narrows area is limited. Pumping occurs in the adjacent 

San Fernando Valley; pumping also occurs several miles 

downstream in the Central Basin (fig. 8). 

Hydrology 19
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Figure 8. Location of precipitation stations, surface-water gaging stations, and ground-water wells in the study area and vicinity.

20 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California
is required to determine the design, feasibility, and operation 

of any inflatable dam. 

Comprehensive, long-term monitoring of water-quality 

along the Los Angeles River has been limited. At two stations, 

USGS 11097500 (at Los Angeles) and USGS 11097490 (at 

Feliz Blvd.), monthly chemical data (nutrients, major ions, 

temperature, dissolved oxygen, and turbidity) were collected 

during the period 1966-73 and 1973-77, respectively. 

 Several agencies have water-quality monitoring programs 

for dischargers and municipal agencies to satisfy either State 

or Federal regulations, State of California Title 22, National 

Pollutant Discharge Elimination System (NPDES), or as part 

of Superfund cleanup requirements. The Los Angeles River 

watershed has impaired water quality in the middle and lower 

parts of the basin owing to runoff from dense clusters of 

commercial, industrial, residential, and other urban activities. 

The U.S. Environmental Protection Agency (USEPA) 303(d) 

list of impairments, due to point and nonpoint sources, 

includes the following: pH, ammonia, selected metals, algae, 

coliform, trash, scum/foam, oil, selected pesticides, and 

selected volatile organic compounds (VOC).

Selected water-quality monitoring in the river and its 

tributaries is being conducted by the City and County of 

Los Angeles and other parties (Michael Mullin, City of Los 

Angeles, unpub. data, 2004). Status and trend monitoring 

involves regular sampling from 17 stations on the river and 

its tributaries for heavy metals and bacteria. In addition, there 

is extensive monitoring of discharge into the Los Angeles 

River from the three water reclamation plants (WRPs)—the 

Glendale, the Burbank (see fig. 8) and the Tillman (about 13 

mi northwest) WRPs. Monitoring of effluent at these locations 

includes VOCs, pesticides, metals, nitrogen, and bacteria. A 

high proportion of inflow to the proposed El Pueblo Lake at 

the confluence would originate as discharge from these WRPs. 

Measurements of depth to ground water in the study 

area are collected from a combination of observation 

wells, abandoned production wells, or shallow site-specific 

monitoring wells by the City of Los Angeles, Department 

of Water and Power, the Los Angeles County Department 

of Public Works, and other entities. Wells in the Glendale 

Narrows typically are perforated in shallow, unconsolidated 

deposits. Less is known about ground-water conditions (water 

levels and quality of water) in the deeper zones, as well as in 

the consolidated basement materials (see Tpss, fig. 4). 

Computer models of varying scales have been constructed 

to simulate and quantify the movement of surface or ground 

water in the Los Angeles area. Larger-scale regional groundwater models have been developed for the San Fernando 

Valley (Patricia Kiechler, Upper Los Angeles River Area 

Watermaster Services, oral commun., 2004), the Raymond 

Basin (Anthony Zampiello, Raymond Basin Management 

Board, oral commun., 2004), and the Central and West Coast 

Basins (Reichard and others, 2003). Smaller-scale groundwater models focus on specific areas such as Taylor Yard 

(Miller Brooks Environmental, 2002). The Southern California 

Coastal Water Research Project (SCCWRP) is currently 

developing a watershed model for the Los Angeles River 

as part of a project studying the southern California coastal 

watershed (Southern California Coastal Water Research 

Project, 2004)

Hydrological Issues

Surface-water hydraulics

Effects

Operation of a temporary lake would affect the 

hydraulic regime of the Los Angeles River both upstream 

and downstream from the dam. However, as planned, El 

Pueblo Lake would not extend into the unlined part of the 

Los Angeles River, and the magnitude of the effects are likely 

to be small given the small volumes of water that would be 

impounded. 

Information on non-winter flows in the Los Angeles 

River and the Arroyo Seco (shown in fig. 9-11) serve as 

examples of the types of analyses that could be undertaken. 

Figures 9 and 10 shows historical maximum daily flow values 

for the Los Angeles River above Arroyo Seco and for Arroyo 

Seco near Pasadena, respectively, for the months of May 

through September, which are the probable months in which 

the dam(s) and lake might be operational. Note that flow 

within the Los Angeles River is much greater than that of the 

Arroyo Seco.

Although median maximum daily values at the Los 

Angeles River above Arroyo Seco for these months are 

generally low—less than or equal to 58 ft3/s for all selected 

months—significantly higher values are shown to occur in 

many years (fig. 9). For the Arroyo Seco near Pasadena, 

median maximum daily values are much lower than in the Los 

Angeles River, generally less than 6 ft3/s. Again, as in the Los 

Angeles River, there is considerable variability from year to 

year.

The number of days with flows exceeding 50 ft3/s at the 

Los Angeles River above Arroyo Seco for the same period 

(May through September) during the year is shown in figure 

11. Prior to about 1978, there were generally very few “dry 

season” days in which daily flows exceeded 50 ft3/s. In recent 

years this flow rate is now exceeded most of the time. This is 

consistent with increasing contributions of effluent discharge 

from upstream wastewater treatment facilities and, to a lesser 

extent, contribution of urban runoff during non-winter periods. 

Information presented in figures 9-11 could be used 

for both the design and the operation of any temporary lake. 

Hydrology 21
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Figure 9. Historical maximum daily discharge at the Los Angeles River above Arroyo Seco (USGS 11097500, LACDPW F57C-R) for May 

through September, 1932-2003.

22 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California
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Figure 10. Historical maximum daily discharge at Arroyo Seco near Pasadena (USGS 11098000) for May through September, 1917-2003.

Hydrology 23
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Figure 11. Number of days with flow exceeding 50 cubic feet per second at the Los Angeles River above Arroyo Seco (USGS 11097500, 

LACDPW F57C-R) for May through September 1932-2003.

24 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California
This type of information has relevance for likely downstream 

discharge and its measuremen, assessment of possible 

sediment load, assessment of contaminant influx, and potential 

changes in upstream operation to make dam operation feasible. 

Evaporation effects should be considered for the 

operation of a temporary lake during summer months. The 

approximate surface area of El Pueblo Lake would be about 

20 acres (4,500 ft by 200 ft). On the basis of pan evaporation 

rates at the closest Los Angeles County evaporation stations 

(Rio Hondo (1014F) and Descanso Gardens (1071B) the 

evaporation for the period May through September may be 

25 to 30 inches (Rodney Brown, Los Angeles County Dept. 

of Public Works, written commun., 2004), which implies 

a season evaporative loss of about 40-50 acre-ft from the 

proposed lake. 

Possible limited removal of channel lining from the 

Arroyo Seco is being considered as part of the Confluence 

Park plan. An evaluation of the potential effects of lining 

removal along the Arroyo Seco has been done (Northeast 

Trees and Arroyo Seco Foundation, 2002). In the context 

of the proposed Confluence Park, issues to consider from a 

surface-water perspective related to channel-lining removal 

include flood response, flow travel time, ground-water 

recharge and discharge (and associated water-quality effects), 

and scour and erosion potential.

Construction of a temporary lake at the confluence will 

alter the flow of sediment through the channels. It is expected 

that particulate materials in the river discharge will settle 

behind the proposed structure, creating a deposit of sediment 

behind the dam. This sediment may influence water quality 

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Figure 12. Relation between river discharge and daily sediment load for the Los Angeles River (USGS 11103000; see page 34) downstream from the study area.

Hydrology 25
and may also provide for a “slug” of sediment that will be 

eroded and transported downstream following annual dam 

deflation. 

Although most of the sediment transport in the Los 

Angeles River and Arroyo Seco occurs following winter 

rainfalls (when sediment is actively eroded from the landscape 

and river channel; Brownlie and Taylor, 1981; Inman and 

Jenkins, 1999), urban runoff and dry-season precipitation can 

introduce sediment to the rivers during the months of dam 

usage. For example, at the Los Angeles River at Long Beach 

(USGS 11103000, approximately 11 mi downstream from 

the confluence), daily sediment flux during periods with flow 

less than 100 ft3/s range from about .7 to 64 tons/day (fig. 12). 

Much of the sediment from these types of flows will deposit 

behind the proposed structure. 

Potential tasks 

1. Conduct an in-depth statistical analysis of historical 

surface-water discharge data. This will be important 

in the design and operation of the proposed El Pueblo 

Lake on the Los Angeles River (see, for example, 

the type of analysis shown in fig 11) and will provide 

guidance on likely effects of any channel-lining 

removal on the Arroyo Seco (Northeast Trees and 

Arroyo Seco Foundation, 2002). 

2. Install or reactivate one or more gaging stations. 

One possible location would be on the Los Angeles 

River just downstream from the proposed lake. A 

second potential location would be at the lower end 

of the Arroyo Seco (to monitor impacts of channellining removal). In addition, seepage runs could be 

conducted along any newly unlined parts of the Arroyo 

Seco in order to quantify losses.

3. Reevaluate historical sediment data. Sediment data 

have been collected at a number of sites within the 

drainages of the proposed project, but the focus was on 

evaluating total sediment discharge, especially during 

winter storms (for example, Brownlie and Taylor, 

1981; Inman and Jenkins, 1999). Since the period of 

interest is the dry season of the year, there is a need 

to reevaluate these historical data for the months of 

interest. Potential changes in watershed management 

also should be considered in this evaluation.

4. Conduct an expanded study of possible sedimentation 

that would occur within the proposed El Pueblo Lake, 

and what would happen to the sediment when the 

dam is deflated for winter flow each year. Develop a 

sediment-transport model, based on and calibrated to 

recent sediment data collected at cross sections above, 

in, and downstream from the lake zone (Bennett, 2001).

Surface-water quality

The overall water quality in the Los Angeles River 

generally is a function of quantity, timing, and source. To 

examine these factors and provide a preliminary description 

of temporal conditions, historical water-quality data collected 

at two gaging stations along the Los Angeles River (USGS 

11097500, at Los Angeles and USGS 11097490 at Feliz 

Blvd.) were compiled. The period of record for these data is 

1966-77, and thus does not represent current river conditions. 

However, the data do provide information on variability and 

patterns, and illustrate the value of reestablishing regular 

water-quality monitoring at gaging stations on the Los 

Angeles River and its tributaries. This is especially important 

because, as shown in figures 9-11, the dry-season flow regime 

has changed considerably over time, mostly owing to the 

increasing volumes of reclaimed water. 

The range and distribution of selected physical 

parameters and chemical constituents are summarized in figure 

13, and the annual variation in dissolved oxygen and specific 

conductance, as well as the relation between dissolved oxygen 

and temperature, is shown in figure 14. 

The boxplots in figure 13 show relatively constant pH 

values, dissolved-oxygen values that range considerably 

(mostly between 2 and 14 mg/L), and temperature values that 

range from 4 to 25 degrees Celsius. Bicarbonate, chloride, 

and sulfate values show a moderate inner-quartile distribution; 

however, all three constituents have extreme values (outliers), 

suggesting different sources of water. Data also indicate that 

surface water in the Los Angeles River has a calcium/sodiumsulfate/bicarbonate composition.

26 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California
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Figure 13. Concentration of selected constituents in surface water from the Los Angeles River, 1966−1977 (USGS stations 

11097500 and 11097490).

Hydrology 27
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The graph in figure 14A illustrates a smooth variation 

in temperature across the calendar year, with a maximum 

corresponding to the summer months when the proposed 

lake would be operational. In general, dissolved oxygen 

concentrations are inversely proportional to temperature, (fig. 

14B); dissolved oxygen concentrations in the river would also 

be lowest during the seasons when the proposed lake would 

be in operation. Closer inspection of figure 14B indicates that 

a few high concentrations of dissolved oxygen (exceeding 

saturation) occurred during non-winter periods (temperatures 

greater than 20 degrees Celsius). This may be explained by 

the occurrence of local algal blooms that may supersaturate 

oxygen in the water. 

Surface water containing the least amount of dissolved 

solids (as indicated by specific conductance values less than 

600 microseimens per centimeter) occurs between the months 

of November and April (fig. 14C). In the adjacent coastal Santa 

Ana basin, lower values are associated with large-volume, 

winter stormflow events (Kent and Belitz, 2004). Higher values 

are more typical throughout the year (including winter periods 

without stormflow events). The effects of increased reclaimed 

water on this relation need to be addressed. 

Given the degree of variability of major-ion 

concentrations in surface-water runoff in the Los Angeles 

River, a similar variation in the range and distribution of other 

water-quality indicators (for example, pesticides, volatile 

organic compounds, organic carbon, and nutrients) is expected 

to occur as well. The concentration and frequency of detection 

of some constituents will be directly correlated with flow, 

while others will be inversely correlated. For example, in 

the Santa Ana river basin, pesticides such as diazinon and 

simazine show a systematic decrease in detection frequency 

for two week period after a storm event. However, pesticides 

were detected more frequently and at higher concentrations 

during and 2-weeks after storms than during dry periods 

(Belitz and others (2004).

Effects 

Surface-water quality will be of critical importance in the 

development of Confluence Park. The creation of a temporary 

lake allows for potential buildup of chemical contaminants. 

In addition, the proposed El Pueblo Lake would increase the 

likelihood of potential exposure (both human and animal) to 

contaminants. In a study conducted on the nearby Santa Ana 

River (Izbicki and others, 2000), samples were collected to 

measure changes in the concentration of selected constituents 

(nutrients, dissolved organic carbon components, selected 

pesticides and volatile organic compounds, and trace elements) 

under different flow conditions and upstream dam operations. 

The suitability of this water as a source of recharge (groundwater supply) to the basin also was evaluated. 

The temporary lake also may influence the microbial 

characteristics of the reservoir water stored and discharged 

from this structure. Discharge from southern California 

streams and storm drain outfalls can be responsible for 

Figure 14. Annual variation in temperature (A); relation between 

dissolved oxygen and temperature (B); and annual variation in 

specific conductance (C) in water from the Los Angeles River, 

1966−1977.

28 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California
degrading coastal water quality, especially owing to the 

introduction of coliform bacteria (for example, Bay and others, 

1999). It is not known how operation of a dry-season lake will 

alter pathogen concentrations of the river water, although the 

structure may serve as a source of pathogens since wildlife 

and people will be attracted to it (for example, Grant and 

others, 2001). If the temporary lake becomes a significant 

source of pathogens, adequate cleanup methods for the water 

(for example, discharge into municipal wastewater treatment 

plants) will be needed. 

Potential tasks 

1. Collect additional data to better assess the suitability 

of surface-water sources for use in El Pueblo Lake 

during summer months, and for potential use as a 

source of ground-water recharge. For both planning 

and operational purposes, it is especially important to 

characterize the seasonal variations in the quality of 

inflow from the Los Angeles River and Arroyo Seco. 

Confluence Park presents an ideal location to establish 

a potential sampling site for a comprehensive waterquality assessment of drainage from the San Fernando 

Valley and Arroyo Seco watersheds. Monitoring should 

include parameters currently being monitored at other 

active sites; more detailed evaluation of the dissolved 

organic carbon, volatile organic compounds, pesticides, 

trace elements, selected isotopes, and emerging 

contaminants (for example, perchlorate, antibiotics, 

endocrine disrupters) also may be appropriate. The 

USGS has established methods for the collection and 

analysis of these types of samples. 

2. In addition to targeted point sampling, continuous 

monitoring of parameters such as specific conductance, 

temperature, dissolved oxygen, turbidity, chlorophyll, 

nutrients, and pH (using a multi-probe sonde) should 

be considered. Monitoring the daily fluctuation of 

these parameters would be particularly relevant for 

understanding the potential for algal blooms in the 

proposed El Pueblo Lake. 

3. Conduct a detailed assessment of the water-quality 

changes that could result from the likely increase in 

discharge of recycled water from the upstream Donald 

C. Tillman, Burbank, and Los Angeles-Glendale 

water-reclamation plants (Schroeder, 2003; Anders and 

Schroeder, 2003; Anders and others, 2004). 

4. Design and implement microbial monitoring plans 

for source and lake waters. The complex dynamics of 

microbes in urban water bodies make prediction of 

the influence of the proposed project difficult. Thus, 

pathogen monitoring of the source and proposed 

lake waters is needed. The monitoring data would 

be incorporated into a water management plan that 

addresses the potential for waters impacting river and 

coastal zones (Grant and others, 2001; Izbicki, U.S. 

Geological Survey, written commun., 2004)

Ground-water hydraulics

Effects 

Ground-water levels would probably not be significantly 

affected by the development of a park at the confluence. Los 

Angeles County well 2781 (fig. 8; State of California well 

number 001S014W14E003S) is an active observation well 

near the Arroyo Seco with recorded measurements dating 

back to the 1930s. As shown in figure 15, the water level in 

this well is shallow—about 40 ft below land surface—and has 

not fluctuated substantially over the period of record. This is 

consistent with most water-levels in the surrounding Glendale 

Narrows and is mostly a function of the relatively thin alluvial 

deposits (see “Geology” section), ground-water flow from the 

adjacent San Fernando Valley, proximity to the unlined portion 

of the Los Angeles River, and the lack of significant pumping 

in the nearby area. 

Channel-lining removal along a small section of the 

Arroyo Seco is not expected to have a significant effect on 

ground-water levels, since there is little storage capacity to 

accommodate additional recharge. However, small amounts of 

recharge, depending on the source, could affect ground-water 

quality (see “Ground-Water Quality section”). Collection and 

evaluation of specific hydrologic (water-level, water-quality, 

and temperature-log) data and stratigraphic information would 

be required to make this determination.

To measurably affect ground-water recharge, a significant 

portion of the channel lining would have to be removed. The 

area downstream from the confluence—the Los Angeles 

Forebay in the Central Basin (see fig. 8)—is better suited as a 

potential location for augmentation of ground-water recharge. 

In the Los Angeles Forebay, water levels have declined 

considerably over a large area as a result of intensive pumping 

(Water Replenishment District of Southern California, 2004a; 

Reichard and others, 2003). 

 Potential tasks

1. Collect and compile water-level, water-quality, and 

temperature-log measurements at selected existing wells to 

provide a preliminary baseline for evaluating potential changes 

in the ground-water flow system that could result from the 

development of the confluence area. 

2. Install new multiple-well monitoring site(s) to 

facilitate the collection of basic hydrologic and detailed lithostratigraphic data with good depth control. Although several 

wells already are present in the study area, most are shallow 

(less than 50 ft), and do not, by themselves, provide the 

means to adequately describe the three-dimensional groundwater flow system. The multiple-well monitoring site(s) will 

provide new detailed data on lithology, physical properties, 

Hydrology 29
water levels and water quality. Several such sites have already 

been constructed in the adjacent Central Basin (Land and 

others, 2002); most have been outfitted with a data logger 

to record water-level changes in each aquifer zone (Water 

Replenishment District of Southern California, 2004b). 

3. Reevaluate the hydrogeology of the confluence area. 

Utilize newly collected subsurface data (gathered from the 

task above) and sequence stratigraphic methods to correlate 

the unconsolidated deposits and develop a sequence-based 

framework for quantifying ground-water/surface-water 

interactions and identifying potential contaminant pathways 

(Hillhouse and others, 2002).

4. Develop a ground-water flow model to quantitatively 

assess the ground-water effects of alternative scenarios for 

lake design and channel-lining removal. A promising groundwater modeling approach would be to incorporate the updated 

MODFLOW (Harbaugh and others, 2000) packages to more 

rigorously consider stream-routing contaminant transport 

and ground-water-lake interaction (Merritt and Konikow, 

2000; Prudic and others, 2004). Such modeling should 

consider different scenarios: presence/absence of a dam; 

extent of channel lining; and different inflows from both the 

Los Angeles River and the Arroyo Seco. Recent modeling 

applications of this type for a regional cross section typical of 

non-humid portions of the western United States have been 

completed by Constantz and Essaid (2003). On a larger scale, 

there would be benefits for developing a regional model that 

incorporated the upper basins (San Fernando and Arroyo Seco 

watersheds), the Glendale Narrows, and the lower coastal plain 

basins (Central, Hollywood). An important use of this unified 

model would be to better quantify the ground-water flux 

through the Glendale Narrows.

Ground-water quality

Effects 

Creation of the proposed El Pueblo Lake as currently 

envisioned (City of Los Angeles, 2004) will likely have little 

effect on ground-water quality. However, any channel-lining 

removal within the Arroyo Seco may affect ground-water 

recharge, and, hence, ground-water quality. In addition, 

if development of Confluence Park is coupled with the 

implementation of new artificial recharge facilities in adjacent 

areas (see “Potential tasks” below), ground-water quality may 

be affected. 

In either case, a network of monitoring wells and a 

sampling program would need to be established to assess the 

effect and magnitude of these changes.

Contaminants in shallow ground water in the Glendale 

Narrows may affect the Confluence Park development by 

degrading the quality of water to the proposed El Pueblo 

Lake (Santa Monica Mountains Conservancy, 2003; 

Environmental Resources Management, 2000). The presence 

and continued input of these contaminants may adversely 

affect the ecosystem health (see “Ecotoxicology” section). For 

example, a survey of organic compounds and heavy metals 

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Figure 15. Water-level hydrograph for well 2781 (State of California well number 001S014W14E003S) in the study area.

30 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California
in streambed sediment, reservoir sediment, and fish tissue 

collected at sites along the Santa Ana River by Burton (2002) 

showed more frequent detections at urban sites than nonurban 

sites. In addition, these data provide a historical record of 

accumulation of pesticide, organochlorine and semivolatile 

organic compounds. 

Potential tasks 

1. Assess the occurrence and distribution of contaminants 

in the ground-water system through compilation of 

existing data and sampling of new and existing wells 

in the Glendale Narrows. Besides characterizing the 

vertical distribution of contaminants, the collection of 

chemical data could address additional questions:

a. What is the source(s) of ground-water recharge 

(local rainfall, urban runoff, stormwater, wastewa-

ter, underflow from an adjacent basin)?

b. Are distinct ground-water flow paths discernable? 

If so, what is the susceptibility of ground water in 

deeper aquifer zones of the Glendale Narrows area 

to contamination?

c. Are decreases in contaminant concentrations 

driven by natural bioremediation processes or by 

dilution?

2. Install a multiple-well monitoring site(s) (see 

“Ground-Water Hydraulics” section) to facilitate the 

collection of water-quality data from depth-specific 

aquifer zones. This would also provide information on 

ground-water conditions in the deepest unconsolidated 

deposits and underlying consolidated deposits for 

which little information currently is available. The total 

borehole depth, depending on location (fig. 4), would 

be as much as 300 ft.

3. Conduct a one-time collection of streambed-sediment 

cores in the unlined portion of the Los Angeles 

River for selected constituents of concern. Sediment 

accumulated in the channel at the end of the dry season 

also should be sampled and analyzed. The occurrence 

of these constituents in fish (or other animal) tissue 

also should be measured and included as part of any 

ecosystem health assessment (see Burton, 2002).

4. Evaluate strategies for operating the proposed El 

Pueblo Lake in conjunction with potential future 

projects for improving water-quality and enhancing 

ground-water recharge downstream, such as: 

(1) diverting water into spreading ponds adjacent to the 

site or to injection wells in the Central Basin, 

(2) unlining and widening portions of the Los Angeles 

river channel.

Hydrology 31
Biology

Biologic overview of the study area

Biological setting

Southern California is home to a rich diversity of native 

and introduced biota. In this arid region, riparian systems, 

such as the Los Angeles River and Arroyo Seco, provide 

narrow bands of habitat within which a disproportionate 

diversity of species reside for part or all of their life cycle. In 

coastal southern California, flood control and urbanization 

have massively altered most native riparian ecosystems, even 

confining them within concrete-lined channels. The result has 

been the reduced distribution and abundance of many species 

native to southern California. For many native species, habitat 

alteration has resulted in their inclusion on lists of imperiled 

species. The Least Bell’s Vireo, Southwestern Willow 

Flycatcher, Arroyo Toad, Santa Ana Sucker, and Unarmored 

Threespine Stickleback are some of the federally listed 

species native to the streams and riparian corridors of southern 

California.

Developments as part of the proposed Confluence Park 

could affect the habitat suitability for native species in this 

region. Although the Los Angeles River’s highly altered 

nature and urban setting make restoring anything similar 

to the original system especially challenging, even small 

enhancements would represent a relatively large improvement 

over current conditions. In the area proposed for Confluence 

Park, upland habitats are now completely paved, and the 

Arroyo Seco and Los Angeles River are confined within 

concrete-lined channels. Any naturalization of this area would 

increase habitat suitability for plant and animal species. 

Ecological restoration projects such as that being considered 

for Confluence Park are reclaiming habitats in cities around 

the world. In many cases, these projects additionally provide 

opportunities to (1) learn how to achieve restoration goals and 

(2) test theoretical models of ecological functioning in the 

process. 

Biological monitoring and modeling

The collection of scientific monitoring data is essential 

to rigorously document the changing status of environmental 

resources. The USGS is involved in a number of biological 

research and monitoring projects in the Los Angeles region. 

The project closest to the confluence area is at Debs Park, 

which is situated less than 2 mi upstream along the Arroyo 

Seco (fig. 16). At this and other sites in the region, USGS 

biologists and cooperators are monitoring populations of 

reptiles and amphibians to better understand their interactions 

with human-dominated landscapes. Farther upstream in 

tributary creeks of the Angeles and San Bernardino National 

Forests, USGS biologists are studying endangered amphibians 

and fishes to determine the causes of endangerment and to 

assist with planning for their recovery. 

The locations of nearby USGS biological research sites 

during the past five years are shown in figure 16. This map 

also includes a simplified view of regional land cover, to 

illustrate the extreme degree to which this region has been 

urbanized. On this map, the pink shaded areas represents 

developed land uses identified as residential, commercial, 

industrial, or transportation; scrub, forested upland, shrubland, 

grassland, or recreational land use dominate the green shaded 

areas. 

As efforts to restore habitats within urban areas spread, 

monitoring is needed to track progress, guide ongoing actions, 

and identify programs that are not working. Until recently, 

conservation and restoration projects were evaluated primarily 

on aesthetics and the assumption that doing something 

was better than doing nothing. This approach is changing; 

increasingly, practitioners must document the results of their 

work (Christiansen, 2003, 2004). Although a biological 

monitoring program adds some cost to restoration, it also 

establishes credibility and allows objective assessment of 

what works, and what does not, and suggests opportunities for 

improvement (Block and others, 2001). Whereas monitoring 

facilitates after-the-fact evaluation of resource status and 

program progress, ecological modeling in advance to project 

implementation helps create plans that are more likely to 

succeed.

An example of ecological modeling in support of 

conservation planning is recent work on mountain lions and 

bobcats in southern California (Hunter and others, 2003). 

That project used habitat models to identify remaining 

regions of suitable habitat, based on habitat preference 

studies and region-wide GIS habitat maps. It identified 

missing or threatened habitat linkages and provided land 

managers with a tool allowing them to focus habitat 

acquisition and management efforts in the most critical 

areas. In another project, biologists evaluated the potential 

impacts on the endangered arroyo toad of a range for dam 

management options. For this risk assessment, the model 

was a representation of the species life history. Comparing 

the life history model with management options revealed that 

managers should avoid dam releases when the greatest number 

of egg masses and tadpoles are most likely to be present 

(Madden-Smith and others, 2004). Moving beyond trial and 

error towards true adaptive management requires the adoption 

of an ecological modeling perspective during the formulation 

of plans. Collection of monitoring data before and after project 

implementation allows managers to evaluate if the system 

responded to treatments in the ways suggested by the model, 

and, where necessary, to refine the model.

32 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California
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Figure 16. Watershed boundaries, selected stream gages, and U.S. Geological Survey biological research monitoring sites.

Biology 33
wide range of species potentially utilizing these habitat 

areas, a variety of survey methods would need to be 

applied.

2. Develop a Geographic Information System (GIS) based 

model of potential changes in habitat connectivity 

along the Los Angeles River and the Arroyo Seco 

caused by proposed park development. Methods exist 

for modeling regional habitat connectivity on the basis 

of species habitat requirements (see Hunter and others, 

2003). Models should address effects on a range of 

species that have differing habitat requirements (for 

example, coyotes, western toads, steelhead, arroyo 

chubs).

3. Conduct long-term monitoring of wildlife traffic 

through the Los Angeles River and the Arroyo Seco 

channels before and after creation of the Confluence 

Park. Monitoring should be designed to test specific 

hypotheses about the value proposed design elements 

have in facilitating or blocking wildlife movement. 

Obtaining baseline data before habitat is altered is 

essential for demonstrating the relationship between 

design elements and habitat use by wildlife.

Wildlife Colonization/Reintroduction

Effects 

As habitats in urban areas are restored to more natural 

states, some species will immigrate and establish populations. 

However, other species of interest may not be able to colonize 

naturally (Trenham and Marsh, 2002). For those species, 

if project goals include the establishment of these species, 

intentional reintroduction may be necessary. For example, the 

Black-bellied slender salamander is a species that is abundant 

throughout residential Los Angeles, but because it is a poor 

disperser it is unlikely to colonize the restored upland habitats 

of Confluence Park. The western fence lizard and various 

flightless invertebrates (for example, scorpions, Jerusalem 

crickets, darkling beetles) also are common regionally, but 

have limited dispersal capacities. Table 1 lists the dominant 

native plants found along the Los Angeles River corridor, and 

table 2 lists vertebrate and invertebrate species that occur in 

natural areas near the project site and the types of habitat they 

usually occupy.

There is great potential for use of the proposed park site 

as a laboratory for wildlife reintroduction in an urban setting. 

Relocations of wildlife are common, but projects that attempt 

to develop standard procedures for reintroduction, and which 

critically evaluate the success of these efforts, are rare. Some 

important questions that wildlife reintroductions should 

address include (Block and others, 2001; Semlitsch, 2002) 

• How many individuals of what life-history stage will 

be released?

Biological Issues

Wildlife Corridors 

Effects

Large areas of southern California have become 

urbanized, creating isolated or weakly connected patches of 

native habitat (for example, urban parks, urban canyons). 

Over time, as chance events lead to local extinctions, 

biological diversity in isolated habitat patches declines, and 

without functional corridors there is little hope for natural 

reestablishment of new populations. Restoring habitat 

corridors through urban areas can improve this situation by 

increasing connectivity among remaining and restored habitat 

patches. In southern California, biologists have already 

attempted to identify corridors that facilitate dispersal among 

habitat patches, and locations where compromised connections 

may be restorable. Most related studies have focused on 

medium to large mammals such as bobcats and mountain 

lions (Hunter and others, 2003). Bolger and others (2001) 

also assessed the use of coastal sage scrub habitat strips by 

birds and small mammals. In fragmented landscapes, habitat 

corridors are of key importance for maintaining widespread 

wildlife populations, especially for species such as large and 

small mammals, reptiles, and amphibians. Riparian areas, in 

addition to linking natural hydrological systems, commonly 

provide the best opportunities for wildlife movement through 

urbanized areas.

Although the Los Angeles River and the Arroyo Seco are 

highly altered systems, they nevertheless serve as corridors 

for dispersal, as shown by recent reports of mountain lions 

in Griffith Park. Animals such as foxes and coyotes also may 

move deliberately through these corridors; winter floods may 

be responsible for the involuntary downstream dispersal of 

some plants and animals. If individuals reach suitable habitat, 

they may survive and establish populations. Restoration of 

upland habitats in conjunction with the creation of Confluence 

Park could increase the area of suitable upland habitats in the 

vicinity. In addition, if animals disperse in a “stepping-stone” 

fashion, creation of even small patches of naturalized habitat 

may enhance dispersal disproportionately. The creation of 

the proposed El Pueblo Lake along the Los Angeles River, 

however, could seasonally eliminate or dramatically reduce 

habitat connectivity for upland and aquatic species that move 

through the river channel (for example, coyotes and fishes). 

Potential tasks 

1. Monitor existing and future restored upland areas for 

colonization by wildlife, especially non-flying species. 

Natural recolonization of upland areas will provide 

information regarding which animals are using the 

river channels as movement corridors. To detect the 

34 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California
• Where is an appropriate source population? 

• Will relocated individuals attempt to return home or 

will they settle in the restored area? 

• How will project success be measured?

A decision to attempt an introduction should reflect a 

match between (1) the species habitat needs and (2) the extent 

of habitat present. Habitat area is at least as important as the 

type and condition of the habitat available. A novel use of 

reintroduction would be as a measure of restoration success, 

with a monitoring program instituted to verify population 

establishment and growth. Although the translocation of 

organisms holds great potential promise as a method for 

restoring ecological systems, caution is warranted to avoid 

detrimental impacts to desirable organisms already occupying 

the release site. Potential risks include introduction 

of disease, parasites, and maladaptive genotypes. 

It is critical that organisms only be translocated 

to sites without extant conspecific populations (if 

a population exists already, habitat improvements 

should be sufficient to increase numbers). Because 

many species differ genetically from location to 

location, at a minimum the source of organisms 

should be as close to the release site as possible. 

However, ideally, a genetic study would be 

conducted to identify the best potential source 

population.

Potential tasks 

1. Inventory and monitor plants and small 

animals for 2 to 4 years following habitat restoration. 

Determine species that colonize restored habitat areas, 

and how their densities change over time. 

2. Use wildlife-habitat relationship models to determine 

species that potentially could be supported on-site. 

If priority species cannot be supported by the site 

as currently configured, then revision of restoration 

design may be needed to remedy this situation. 

3. Introduce and monitor populations of desirable species 

that inventories have shown to be absent from the 

site. Species should be selected on the basis of an 

assessment of the available habitats, space, and other 

requirements (Task 2). Ideally introductions should 

be conducted as experiments to learn how to better 

conduct future introductions, and to test theories and 

assumptions regarding the species biology, or both.

4. Consider using topsoil relocation as a strategy for 

establishing a diverse community of native plants. 

Topsoil can be obtained from nearby sites slated for 

development. This method has been shown to be more 

successful than seeding in other systems. If there is 

evidence or concerns that existing on-site soils are 

contaminated, this would also be a potential remedy.

Botta’s pocket gopher

Harvest mouse

Pacific slender salamander

Western fence lizard

Native species in the study area (see table 2)

Biology 35

Western toad
Scientific Name Habitats Where Each Species Was Observed

N In Water Wet Sandy Deeper Wet 

Soils

Moist Gravel 

Bars

Dry Gravel 

Alluvium

Elevated 

Gravel 

Banks

Salix spp. 12 X X X

Baccharis salicifolia 9 X X X X

Ludwigia peploides 7 X

Platanus racemosa 6 X

Xanthium strumarium canadense 6 X X

Bidens laevis 5 X X

Datura meteloides 5 X X

Populus fremontii 5 X

Alnus rhombifolia 4 X X

Helianthus annuus lenticularis 4 X X X

Heterotheca grandiflora 4 X X

Lemna minor 4 X

Scirpus acutus 4 X

Polygonum hydropiperoides 3 X

Typha latifolia 3 X X

Ambrosia acanthicarpa 2 X

Artemisia douglasiana 2 X

Cucurbita foetidissima 2 X

Cyperus odoratus 2 X

Eleocharis parishii 2 X

Eriogonum fasciculatum 2 X

Lepidospartum squamatum 2 X

Malacothrix saxatilis v. tenuifolia 2 X

Mimulus cardinalis 2 X X

Physalis philadelphica 2 X

Polygonum amphibium 2 X X

Scirpus americanus 2 X X

Senecio douglasii 2 X

Acer negundo 1 X

Chrysopsis villosa 1 X

Epilobium ciliatum 1 X

Eremocarpus setigerus 1 X

Gnaphalium microcephalum 1 X

Leptochloa uninervia 1 X

Lotus scoparius 1 X

Malosma laurina 1 X

Melica imperfecta 1 X

Rumex salicifolius 1 X

Sambucus mexicana 1 X

Scirpus validus 1 X

Yucca whipplei 1 X

Table 1. Native vascular plants documented at sites along the Los Angeles River. 

[N = number of sites out of 14 visited where that species was observed (Wallace, 1993). General habitat types where species were found: in standing or moving 

water; wet sandy areas away from channel; deeper soils but at edge of water; deeper moist soils permanent gravel bars; dry, at least on surface, gravel alluvium; 

and dry elevated gravel banks and concrete banks]

36 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California
Non-Native and Pest Species

Effects

Prominent among the threats to California’s native 

biodiversity is the introduction of species from outside of the 

region. Non-native species often establish thriving populations 

that compete with natives for resources or prey on them 

directly. Abundant non-native species pose a challenge for 

restoring native communities, a challenge which is even 

greater in urban areas (Weiss, 1999). Non-native plants often 

invade disturbed habitats and can alter system conditions 

to the point where the habitats are unsuitable for native 

plants and animals (for example, vast ice plant and giant 

reed monocultures). In urban aquatic systems, the persistent 

release of pet animals (for example, turtles and fish) makes 

eliminating those species virtually impossible (Spinks and 

others, 2003). Fortunately, the distribution and abundance 

of most non-native species can be controlled with regular 

“weeding” programs, typically educating local volunteer 

workers in the process. Providing educational materials and 

outreach through pet stores, nurseries, and schools may also 

reduce future releases. 

 In addition to concern for native species, habitat 

restoration projects must also consider public health and 

safety. Probably the largest public health concern relative 

to this project will be worries that mosquito populations, 

and thus increased disease transmission, may be enhanced. 

During calendar year 2004, Los Angeles county had 328 

confirmed cases of West Nile Virus with 13 related deaths 

(State of California, 2005). Presently the channelized portions 

of the Los Angeles River and Arroyo Seco contain relatively 

few areas of standing water where mosquitoes breed, and 

restoration efforts should avoid creating new breeding sites. 

Rats are the other major pest that any project design should 

seek to discourage.

Potential tasks 

1. Inventory and monitor study area to determine the 

presence and population densities of invasive species 

of plants and animals. 

2. Conduct studies to determine the level of effort and 

frequency of weeding/trapping/poisoning programs 

needed to restrict populations of non-native and pest 

species at an acceptable density (Keitt and others, 

2002).

3. Create educational materials to inform visitors of local 

biological issues. Signage explaining how the release 

of domestic animals can massively disrupt natural 

systems, could help minimize future introductions. 

Presenting visitors with a clear vision of the desired 

Table 2. Native species with potential to establish resident populations within the proposed Confluence Park.

[Asterisks denotes species captured by USGS at Ernest E. Debs Regional Park which is less than 2 miles upstream and adjacent to Arroyo Seco; pound sign 

denotes species reported by others at Debs Park.]

Common Name Basic Habitat Associations

* Western skink Scrub/Grassland/Forest

* Western fence lizard Generalist/Perches

 California legless lizard Sand/Loose Soil/Litter

* Western whiptail Dense Scrub/Loose Soil

 Side-blotched lizard Open/Rock/Sand

# Southern alligator lizard Generalist/Open Canopy

* Ringneck snake Forest/Scrub

# Pacific slender salamander Forest/Scrub

 Pacific chorus frog Breeds in Slow Moving Water /Generalist

# Western toad Breeds in Slow Moving Water /Generalist

* Harvest mouse Dense Grass/Scrub

* Botta’s pocket gopher Open Grass/Scrub/Forest

 Arroyo chub Sand or Mud Bottom/Slow Water

 Jerusalem cricket Loose Soil

 Darkling beetle Generalist

 Scorpion Surface Cover Items/Generalist

Biology 37
future condition of the local system could also help 

them understand why certain restrictions exist and win 

over advocates.

4. Determine the best management policies for operation 

of the proposed El Pueblo Lake to minimize mosquito 

problems.

Ecotoxicology

Effects

Past land-use effects and on-going contamination present 

additional challenges for habitat restoration in urban areas. It 

should be recognized that chemical contamination will likely 

limit the pool of species capable of surviving in the restored 

habitats. Failure of plant or wildlife populations to thrive 

within the project area may be due to contamination rather 

than the absence of suitable habitat. Water present in aquatic 

habitats in urban settings commonly originates from storm 

runoff, discharge from wastewater treatment facilities, and 

urban runoff (surface water quality is discussed in detail in the 

“Hydrologic Issues” section). Areas currently paved may have 

contaminated soils, and substances from nearby contamination 

could potentially migrate onsite over time. Finally, nonpoint sources such as automobiles can contribute persistent 

detrimental chemical inputs such as heavy metals, organics, 

and nitrogen (Weiss, 1999). 

Potential tasks 

1. Before, during, and after redevelopment construction, 

conduct regular sampling of aquatic benthic 

invertebrates in the study area.

2. Conduct bioassays or analytical analyses of upland soil 

toxicity. Rearing small animals such as salamanders 

or earthworms in soils from the site may lend insights 

into the capacity of the site to support certain species.

3. Consider the inclusion of off-channel pools that fill 

with rainwater and local runoff, as part of the habitat 

restoration. These pools would be less subject to 

runoff-borne contamination and thus might provide 

important aquatic habitat for wildlife. 

Options for Restoration of Local Habitat and Ecology

The various design elements that are either currently 

being considered, or that could be considered in the future, 

will affect the local habitat and ecology in different ways. 

Recognizing that not all suggested restoration elements 

might be implemented or even feasible, they are considered 

independently. 

Effects

Temporary lake— Birds, bats, and flying insects are 

attracted to inundated areas as sources of food and water. 

Insects and some amphibians could possibly use a temporary 

lake as breeding habitat; however, because the lake will not 

be permanent, breeding populations of fully aquatic species 

will not become established. As mentioned previously, the 

movement of some animals (for example, coyotes and fishes) 

along the Los Angeles River corridor would be reduced or 

eliminated during the months when the lake is full, reducing 

the potential for colonization of nearby upland habitats. The 

impediment to movement of terrestrial organisms might be 

reduced by providing as wide an undeveloped upland buffer 

as possible along one side of the lake. Two additional effects 

to consider are the potential establishment of non-native/pest 

species and poor water-quality conditions. Because most 

non-native aquatic pests (for example, bullfrogs, largemouth 

bass, red-eared slider turtles) require a permanent source of 

water, their establishment is unlikely. Additionally, as a result 

of non-point source contributions and upstream contamination, 

water quality in the temporary lake may not be suitable for 

human use and wildlife. Finally, with emerging diseases like 

West Nile Virus, alterations that might enhance mosquito 

populations may meet opposition. 

Removal of concrete from channel bottoms— Unlined 

portions of the Los Angeles River, just upstream from the 

confluence (fig. 2), support riparian vegetation. Channellining removal on parts of the Arroyo Seco could help to 

create a more natural streambed. This would replace the 

current homogeneous flat concrete bottom with a mildly 

meandering low-flow stream course, possibly including a 

heterogeneous mix of runs and pools. The first result would 

be the reestablishment of riparian vegetation in the stream 

channel. A variety of wildlife species are also likely to benefit. 

Birds may feed, roost, and nest in new riparian vegetation. 

Butterflies and other terrestrial invertebrates would benefit 

from increased riparian vegetation. Areas of open water would 

likely attract wading birds and waterfowl. Amphibians may 

breed in off-channel pools, and there is even potential for the 

creation of aquatic habitat suitable for the native Arroyo Chub. 

The increased vegetative cover and a more natural habitat 

structure would also enhance the value of these channels as 

wildlife corridors.

Concerns that would have to be addressed if lining 

removal is implemented include the proliferation of invasive 

species. Because giant reed (Arundo donnax) is likely to 

colonize the restored creek channel, a weed management plan 

would be an essential project component. Pools in the creek 

may become infested with crayfish, mosquito fish, and other 

exotics; these also could be reduced by persistent removal 

with minimal effort. In addition, attracting wildlife may 

be detrimental to individual health and the maintenance of 

resident populations if water quality is poor. 

38 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California
Replacement of vertical channel walls— At the 

confluence, flow in the Los Angeles River and the Arroyo 

Seco is contained by concrete channels walls that, in some 

areas, are vertical and more than 10 feet high. For most nonflying vertebrates, such as mice, toads, and certain insects, 

these walls likely prevent movement between the riparian 

corridor and upland habitats. One possible expansion of 

currently considered plans for the Confluence Park could be 

to replace the vertical walls with more mildly sloped concrete 

banks on the Arroyo Seco (Northeast Trees and Arroyo Seco 

Foundation, 2002). Further consideration of this option would 

require engineering analysis to determine feasibility. Sloping 

walls would allow a greater diversity of non-flying vertebrates 

to move back and forth across the channels and between 

upland habitats. A design that allows vegetation to establish 

along the banks would provide cover and food for animals 

and a more natural transition between the stream and upland 

habitats. Riprap construction might also provide habitat or 

temporary refuge for small animals. Wildlife species benefiting 

from this design would include invertebrates, reptiles, 

amphibians, and small mammals; of potential concern is that 

the habitat for rats may also be enhanced. 

Establishment of native upland vegetation/gardens— 

Currently this area supports essentially no plant or animal 

life, so any revegetation would be an improvement. 

For the revitalization of upland habitats, the planning 

documents suggest a variety of alternatives including 

some reestablishment of native vegetation. Any attempt at 

revegetation would begin with the removal of pavement. Then 

the rehabilitation or removal of the underlying soil may be 

necessary if contamination has occurred. Native plants and 

topsoil may be obtainable from nearby sites scheduled for 

development. Introducing plants along with their native topsoil 

may speed the establishment of other species commonly 

missing from restoration project sites, such as annual plants, 

soil insects, worms, microbes, and fungi. Native plants 

recorded in surveys of 14 sites along the Los Angeles River 

(Wallace, 1993) are summarized in table 1.

View of the Arroyo Seco entering the Los Angeles River (photograph by Dale Cox, U.S. Geological Survey)

Biology 39
Although native gardens would likely support more 

native animals, designs with non-natives will also enhance 

wildlife habitat. Gardens in similar settings harbor a variety 

of native animals including birds, insects, bats, lizards, 

salamanders, and small mammals. If the vertical walls along 

the Arroyo Seco were removed, western toads and Pacific 

chorus frogs may also visit the restored uplands. 

Concerns for the success of upland restoration 

include potentially contaminated soils, physical barriers 

to recolonization, and space limitations. Plants may not be 

capable of surviving onsite if the soil is contaminated or 

nutrient-poor. Finally, the capacity of upland areas to support 

resident populations may be limited by the small area of 

habitat that will be available. Benefits to the public include 

enhanced recreation opportunities, plant and wildlife viewing 

opportunities, and reduced storm runoff from the site.

Enhancement of bat roosts— Bats are conspicuous 

inhabitants of many urbanized areas. Bats fall into two major 

groups: tree roosting species and crevice roosting species. 

These species are commonly found in many cities because 

high densities of large trees are routinely maintained as part 

of the urban landscape (Gehrt and Chelsvig, 2003, 2004). In 

southern California the red bat and the hoary bat are common 

tree-roosting species, and, because these animals do not 

roost communally, the addition of trees would benefit the 

population. In comparison, many crevice roosting species are 

communal roosters and have been observed to aggregate in 

large numbers beneath bridges and within other structures. 

Common crevice-roosting bats in the region include the big 

brown bat, the Yuma Myotis bat, and the Mexican free-tailed 

bat. 

Recent studies have indicated that some bats are even 

more abundant in urban areas than neighboring agricultural 

landscapes, most likely because large urban trees and 

man-made structures provide essential roosts (Gehrt and 

Chelsvig, 2003, 2004). In Austin, Texas, between March 

and November an estimated 1.5 million Mexican free-tailed 

bats emerge nightly from their roosts beneath the Congress 

Avenue bridge; an estimated 100,000 people visit the site 

annually to witness this phenomenon. In addition to serving 

a general public interest in wildlife, a healthy bat population 

would help suppress pest insects, making the proposed park 

a more pleasant setting (Agosta, 2002). Planting native trees 

such as cottonwoods and sycamores in the confluence area 

would likely benefit species such as the red and hoary bats; 

the numerous bridges in the vicinity of the confluence likely 

support crevice nesting species, which would further benefit 

from the naturalization of the surrounding area.

Potential tasks 

1. Initiate a biological inventory and monitoring program 

to document the current status of biota in the project 

area. If a goal is to remove part of the concrete lining, 

initial inventories should focus on the targeted area and 

also the currently unlined sections of the Los Angles 

River upstream of the project area. The collection of 

baseline data and monitoring to document changes 

would help track progress towards project goals. Any 

monitoring work associated with this project should be 

coordinated with existing restoration studies upstream 

and in the headwater areas. 

2. Evaluate feasibility and potential benefits of 

widening the river channels and creating sloping 

banks. Although replacing vertical concrete walls on 

both sides of the channel would best mimic natural 

conditions, replacing only the northern wall of the 

Arroyo Seco would achieve many of the same benefits.

3. Conduct post-development monitoring to evaluate 

project success in restoring habitat for riparian birds. 

The development of Confluence Park provides an 

opportunity to restore stream habitats for endangered 

and more common riparian birds. The design and 

construction of this restoration phase should be 

followed by data collection and monitoring to evaluate 

project success and to improve future restoration 

efforts.

4. Determine if bats are currently roosting under freeway 

and railroad bridges near the confluence area. If bats 

are present, this habitat value should be maintained or 

enhanced. Promote establishment of large native trees 

for the benefit of birds and bats.

Summary and Discussion

This report has identified a set of science issues that 

relate to the proposed development of a park at the confluence 

of the Los Angeles River and the Arroyo Seco, and has 

delineated a suite of potential work tasks to address these 

issues. The geological, hydrological, and biological issues are 

summarized below. 

Geological issues of concern relate to surface faulting, 

earthquake ground motions, liquefaction, landsliding, and 

induced seismicity. The risk from potential surface faulting 

is considered to be low. There is a potential risk from 

earthquake ground motion because of the proximity of faults 

capable of producing large earthquakes and because of the 

amplification effect of the thin alluvium within the Glendale 

Narrows. The likely risks of liquefaction are uncertain at 

the Confluence Park site. Better determination of these risks 

would require more detailed assessment of ground-water 

levels and physical properties of the alluvial materials. 

Landslides are likely not a hazard for the confluence 

development, nor is induced seismicity. 

Hydrological issues of concern relate to the hydraulics 

and water quality of both surface water and ground water. In 

general, the hydraulic effects related to the Confluence Park 

development are likely to be small given the relatively small 

40 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California
volume of the proposed lake and the fact that no channellining removal is being considered for the Los Angeles River. 

Water-quality effects are more likely to pose concerns. 

Flow in the Los Angeles River during non-storm periods 

has become increasingly dominated by discharge from waterreclamation facilities. This has hydraulic implications for the 

design and operation of a temporary lake. In addition, the 

dry-season flows in the river will deliver sediment that would 

accumulate within the lake. Discharge from reclamation 

facilities also affects the quality of water that would be 

impounded in the proposed lake. Creation of a lake allows 

for potential buildup of chemicals from the facilities and 

from other sources. In addition, the presence of a temporary 

lake increases the potential likelihood of human and animal 

exposure to chemical and microbial contaminants. Historical 

water-quality data for the Los Angeles River indicate 

periods with high dissolved oxygen concentrations during 

warm summer months. This may be indicative of algal 

blooms, which are of potential concern for a temporary lake. 

Ground-water levels would likely be unaffected by a 

temporary lake in the lined portion of the Los Angeles River, 

but could be affected by channel-lining removal along the 

Arroyo Seco. Such lining removal also could affect groundwater quality. Ground-water quality could be affected if water 

from the lake were diverted to other locations for artificial 

ground-water recharge. Current ground-water contamination 

will have impacts on some proposed components of the 

Confluence Park. 

Biological issues of concern include colonizationextinction dynamics, wildlife corridors, wildlife reintroduction, 

non-native species, ecotoxicology, and restoration of local 

habitat and ecology. For all of these issues, a key fact is 

that relatively modest enhancements in habitat values that 

result from the proposed Confluence Park would be a large 

improvement over current conditions. Colonization dynamics 

suggest that newly developed park/garden areas will eventually 

increase the species diversity. Given the small park area, 

however, only animals with small home ranges would be able 

to maintain self-sustaining populations. Many components of 

the proposed confluence area redevelopment would enhance 

the functioning of the Los Angeles River and the Arroyo Seco 

as wildlife corridors. Decisions on wildlife reintroduction in 

the confluence area will need to consider both the type and 

area of habitats required. Restoration of native species in the 

confluence area will be affected by the invasion of non-native 

plants and animals. The limited time period of operation 

of the proposed El Pueblo Lake probably would limit the 

likelihood of establishment of additional riparian and aquatic 

invasive species. Restoration of habitats in the confluence area 

also could be hindered by any chemical contamination that 

limits the group of species capable of surviving.

Individual design options for Confluence Park are 

considered in the context of their potential effect on restoration 

of local habitat and ecology. The temporary lake could attract 

insects and some amphibians that would use it as breeding 

habitat, although the lake would not be filled long enough 

to establish permanent populations of most aquatic species. 

The water quality in the lake could be detrimental to wildlife 

health. Channel-lining removal along the adjacent part of the 

Arroyo Seco could help restore riparian vegetation, which, in 

turn, could provide an effective habitat for a variety of native 

species, as well as invasive species such as giant reed Arundo 

donnax. Replacing any vertical channel walls along the Arroyo 

Seco with more mildly sloped banks would allow a greater 

diversity of species to move between the stream and upland 

habitats. Creating native plant gardens is included as part of 

the Confluence Park plans. Because the site currently supports 

sparse plant and animal life, any revegetation would be an 

improvement.

Table 3 summarizes a list of potential work tasks to 

address the geological, hydrological, and biological issues 

described above. Table 3 provides both a generalized list that 

could apply to a range of stream restoration projects and a 

site-specific list for the Confluence Park project. Note that 

while many of the proposed tasks involve data collection and 

analyses to be conducted prior to implementation of the Park, 

several of the hydrology tasks (2, 8, 9, 10) and most of the 

biology tasks (1, 3, 4, 6, 8, 10, 13) involve ongoing monitoring 

during and after park development. 

While this report has addressed scientific issues 

associated with a specific proposed project, table 3 illustrates 

how many of the specific tasks delineated for addressing 

science issues at Confluence Park can be generalized and 

considered for a range of restoration projects. In addition, the 

overall framework presented here for assessing the relevant 

geologic, hydrologic, and biologic issues associated with 

potential development of the Confluence Park could be applied 

to other stream restoration projects in different physiographic 

settings and at various scales.

Summary and Discussion 41
GEOLOGY 

General tasks for urban stream restoration projects Specific tasks for proposed development of Confluence Park

1A. Utilize seismic-geophysical methods to map relevant faults. 1B. Conduct a shallow, high-resolution seismic-reflection line 

to determine whether faults beneath the Confluence Park 

area in fact displace the Puente Formation and disrupt 

Quaternary strata. 

2A. Conduct trenching or borehole investigations to validate 

the seismic interpretations and to determine recency of 

fault activity and amount of fault movement.

2B. Same as general task 2A.

3A. Incorporate fault information into design of project 

facilities.

3B. Incorporate information on potential surface faulting into 

decisions on the placement of critical facilities in the Confluence Park area.

4A. Derive probable earthquake ground motions using 

industry-standard techniques and incorporate into design 

standards for buildings and engineering works.

4B. Same as general task 4A.

5A. Develop appropriate velocity profile(s) to evaluate 

near-surface shear-wave effects.

5B. Determine a minimum 100-ft-deep shear-wave velocity 

profile for the Confluence Park area to better model 

near-surface site-amplification effects. Shear-wave 

measurements could be made in a borehole drilled for 

additional purposes. 

6A. Assess likelihood of seiche developing in any reservoir 

during an earthquake, and effect on temporary and 

permanent structures. 

6B. Model the proposed El Pueblo Lake to determine the likelihood of a seiche developing in this temporary lake during 

an earthquake. Evaluate the design of the clamps that anchor the portable dams in place to determine if they could 

withstand lateral loads imparted by the oscillations.

7A. Determine liquefaction susceptibility, utilizing cone 

penetrometer soundings, geotechnical boreholes, and 

installation of piezometers.

7B. Conduct a detailed subsurface investigation of the 

Confluence Park area to evaluate the liquefaction 

susceptibility of the underlying sediment. Such an 

investigation should involve the use of cone penetrometer 

soundings, several geotechnical boreholes, and installation 

of new piezometers.

8A. Perform liquefaction-hazard analyses to assess the 

potential problem and as input in facilities design.

8B. Perform liquefaction hazard analyses to assess the

 potential problem and as input in facilities design. 

Incorporate data from cone penetrometer soundings, 

geotechnical boreholes, and new piezometers. 

9A. Conduct slope stability analyses of proposed channellining removal.9B. Conduct slope stability analyses along Arroyo Seco where 

channel-lining removal is proposed. 

HYDROLOGY

General tasks for urban stream restoration projects Specific tasks for proposed development of Confluence Park

1A. Conduct an in-depth statistical analysis of relevant 

historical surface-water discharge data 

1B. Conduct an in-depth statistical analysis of historical surface-water discharge data for the Los Angeles River and 

the Arroyo Seco. 

2A. Evaluate existing surface-water monitoring network. Identify needs for additional gaging stations or more frequent 

data collection to adequately monitor stream restoration 

objectives.

2B. Install or reactivate one or more gaging/sampling stations. 

One possible location would be on the Los Angeles River 

just downstream from the proposed lake. A second potential location would be at the lower end of the Arroyo Seco 

to monitor impacts of channel-lining removal. 

3A. Conduct seepage runs to quantify changes in discharge 

resulting from any modification to the stream channel. 

3B. Conduct seepage runs along any newly unlined parts of the 

Arroyo Seco in order to quantify losses.

4A. Conduct a thorough statistical analysis of historical 

sediment data to quantify likely sedimentation effects of 

restoration activities.

4B. Reevaluate historical sediment data for the months of interest (period of likely operation of El Pueblo Lake). 

Table 3. Summary of potential general tasks for urban-stream restoration projects and specific tasks for the proposed development of 

Confluence Park.

42 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California
HYDROLOGY

General tasks for urban stream restoration projects Specific tasks for proposed development of Confluence Park

5A. If sediment impacts are considered likely, develop a sediment transport model that is based on, and calibrated to, 

recent sediment data collected at cross sections above, in, 

and below restoration area. 

5B. Develop a sediment transport model that based on, and 

calibrated to, recent sediment data collected at cross 

sections above, in, and below the Confluence Park area. 

Evaluate likely possible sedimentation within the proposed 

El Pueblo Lake, and what happens to the sediment when 

the dam is deflated for winter flow each year. 

6A. Collect baseline surface-water quality data and assess 

suitability for proposed activities within the stream restoration area.

6B. Collect additional surface-water-quality data to assess suitability for use in El Pueblo Lake during summer months, 

and for potential use as a source of ground-water recharge. 

7A. Characterize the sources of surface-water flow, and assess 

the potential impact to the overall water-quality resulting 

from changes in the contribution of these sources, including recycled wastewater.

7B. Conduct a detailed assessment of effects of likely increase 

in discharge of recycled wastewater from the Donald C. 

Tillman, Burbank, and Los Angeles-Glendale water reclamation facilities (WRPs). 

8A. Design and implement appropriate microbial monitoring 

plans to compliment the collection of chemical data (see 

Hydrology task 7A). 

8B. Design and implement a microbial sampling/monitoring 

plan to characterize baseline conditions of effluent for the 

proposed El Pueblo Lake and to assess the suitability of 

this source for aquatic species (see Biology task 10B).

9A. Collect and compile water-level, water-quality, and temperature-log measurements in existing wells to establish 

baseline conditions in the stream restoration area. 

9B. In addition to general task 9A, design a network of groundwater sampling sites and collect new data to quantify 

potential changes associated with confluence development, 

such as channel-lining removal along the Arroyo Seco (see 

also general task 11A).

10A. Establish sufficient ground-water monitoring/sampling 

stations to assess surface-water/ground-water conditions 

within the stream-restoration area, and with respect to the 

watershed.

10B. Install new multiple-well monitoring well site(s) in the 

Glendale Narrows area to facilitate the collection of basic 

hydrologic, litho-stratigraphic, and water-quality data with 

good depth control. 

11A. Assess the adequacy of the current geohydrologic characterization of the study area for evaluating ground-water/

surface-water conditions. Update as needed.

11B. Develop a revised hydrostratigraphic framework for the 

Glendale Narrows area utilizing new and existing data.

12A. Develop ground-water model(s) to assess hydraulic effects of different stream-restoration scenarios. 12B. Develop a ground-water flow model to quantitatively assess the ground-water effects of alternative scenarios for 

lake design and channel-lining removal.

13A. Identify threats to water quality and assess the existing distribution of contaminants in ground water in, and 

adjacent to, the stream-restoration area. 

13B. Characterize the occurrence and transformation of selected 

ground-water contaminants of concern through compilation of existing data, and sampling of new and existing 

wells in the Glendale Narrows area.

14A. Assess the distribution of selected contaminants of concern in shallow sediment, and potential effect on stream 

water quality.

14B. Conduct a one-time sampling of streambed sediment and 

selected aquatic-species tissue in the unlined portion of 

the Los Angeles River for constituents of concern (such as 

heavy metals, organochlorines, and semivolatile organic 

compounds.) 

15A. Consider integration of specific restoration activities with 

regional water management activities.

15B. Evaluate new locations for potential construction of new 

artificial recharge projects in adjacent areas to improve 

water quality and enhance ground-water recharge.

Table 3. conintued

Summary and Discussion 43
BIOLOGY 

General tasks for urban stream restoration projects Specific tasks for proposed development of Confluence Park

1A. Monitor species diversity within restoration area to assess 

impacts. 

1B. Monitor the species diversity present in the area over 

time to assess impacts of any channel modification on the 

potential for immigration.

2A. Develop model of habitat and animal traffic for different 

restoration options

2B. Develop a model of the likely effects on habitat connectivity and animal traffic of a range of design options along 

the Los Angeles River corridor in the area of the Confluence Park. 

3A. Monitor wildlife traffic in and adjacent to restoration area. 3B. Monitor wildlife traffic along both the Los Angeles River 

and the Arroyo Seco. 

4A. Inventory and monitor small flightless animals for 2 to 4 

years after habitat restoration to determine what species 

rapidly colonize the site. 

4B. Same as general task 4A.

5A. Use wildlife-habitat relationship models to determine species that potentially could be supported in the study area. 5B. Same as general task 5A.

6A. Conduct and monitor experimental wildlife introductions 

for selected species that have apparently failed to colonize 

on their own.

6B. Same as general task 6A.

7A. Consider using topsoil relocation as a strategy for helping 

to establish a diverse community of native plants. 

7B. Same as general task 7A. 

8A. Inventory and monitor study area to determine the presence and population densities of invasive species of plants 

and animals. Evaluate programs for restricting population 

densities of these species. 

8B Same as general task 8A. 

9A. Create educational materials to inform visitors of local 

biological issues.

9B. Same as general task 9A. 

10A. Monitor contaminant concentrations in the water and the 

corresponding condition of aquatic animals in restored 

habitat area. 

10B. Design and implement a plan—in conjunction with 

Hydrology tasks 6B, 8B, and 14B—for the collection of 

biological and microbial indicators to assess the overall 

health of the Confluence Park ecosystem.

11A. Consider the inclusion off-channel pools that fill with 

rainwater as part of the habitat restoration. 

11B. Same as general task 11A.

12A. Assess biologic impacts of alternative designs for streamchannel geometry.12B. Conduct evaluation to determine the benefits and feasibility of widening the channel bottom within the park. 

13A. Conduct pre- and post-development monitoring to evaluate impacts of project on specific species of concern. 13B. Conduct a post-development monitoring to evaluate 

project success in restoring habitat for endangered riparian 

birds. Determine if bats are currently roosting near the 

confluence area and develop a plan to maintain/enhance 

this habitat. 

Table 3. conintued

44 Geological, Hydrological, and Biological Issues Related to Confluence Park, Los Angeles, California
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Heading 1 49

Acknowledgements

Technical Support

Chuck Arnold (MRCA)

James Bennett (USGS)

Kathleen Bullard (MRCA)

Dale Cox (USGS)

Sean Devlin (USGS)

Robert Fisher (USGS)

Gary Fuis (USGS)

Lucy Jones (USGS)

Cara McClane (MRCA)

Michael Nolan (USGS)

Michael Shulters (USGS)

Lisa Soghor (MRCA)

Walter Swain (USGS)

T’Shaka Toure (USGS)

Editorial, Graphics and Layout

Myrna DeBortoli (USGS)

Tyler Johnson (USGS)

Alice McCracken (USGS)

Laurel Rogers (USGS)

Jerry Woodcox (USGS)

Report Review and Approval

Kenneth Belitz (USGS)

Gary Busteed (NPS-National Park Service)

Thomas Fumal (USGS)

Rick Iwatsubo (USGS)

Charles Kaehler (USGS)

Keith Kirk (USGS)

Gregory Lines (USGS)