Scientific Investigations Report 2011–5121

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2011–5121

Prepared in cooperation with the City of Houston

Relations Between Hydrology, Water Quality, and Taste-andOdor Causing Organisms and Compounds in Lake Houston, 

Texas, April 2006–September 2008

Prepared in cooperation with the City of Houston

Relations Between Hydrology, Water Quality, and Taste-andOdor Causing Organisms and Compounds in Lake Houston, 

Texas, April 2006–September 2008

Apr 

May 

June 

July 

Aug 

Sept 

Oct 

Nov 

Dec 

Feb 

Mar 

Apr 

May 

June 

July 

Aug 

Sept 

Oct 

Nov 

Dec 

Feb 

Mar 

Apr 

May 

June 

July 

Aug 

Sept 

Jan Jan 

Inflow, in cubic feet per second

10

100

1,000

10,000

100,000

Water residence time, in days

0

100

200

300

400

500

600

700

2006 2007 2008

EXPLANATION

Daily mean inflow

Discrete sample collection

Daily mean water residence time

Annual mean inflow

Long-term mean inflow (1984–2008)
Front cover:

Background, Photograph showing a mobile, multi-depth continuous water-quality monitoring station, 

Lake Houston, January 2010. 

Top, Photograph showing field personnel servicing a water-quality-monitoring sonde that is attached 

by a data cable to an automated reel, part of a mobile, multi-depth continuous water-quality monitoring 

station, Lake Houston, May 2008. 

Back cover, Photograph showing field personnel collecting water from Lake Houston using a 

peristaltic pump (pump not shown) through Teflon-lined tubing into various water-quality-sample bottles, 

Lake Houston, April 2006. 
Relations Between Hydrology, Water 

Quality, and Taste-and-Odor Causing 

Organisms and Compounds in Lake 

Houston, Texas, April 2006–September 

2008

By Amy M. Beussink and Jennifer L. Graham

Prepared in cooperation with the City of Houston

Scientific Investigations Report 2011–5121

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

This and other USGS information products are available at http://store.usgs.gov/

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Denver, CO 80225

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Any use of trade, product, or firm names 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:

Beussink, A.M., and Graham, J.L., 2011, Relations between hydrology, water quality, and taste-and-odor causing 

organisms and compounds in Lake Houston, Texas, April 2006–September 2008: U.S. Geological Survey Scientific 

Investigations Report 2011–5121, 27 p. 
iii

Acknowledgments

The authors thank the City of Houston Water Quality Laboratory for providing analytical 

services, and the City of Houston Public Works and Houston Police at Lake Houston for 

providing reservoir access, storage facilities, and security. The authors also acknowledge 

Timothy Oden, Michael Turco, Jeannette Oden, and Michael Burnich for technical guidance 

and assistance with data-collection activities throughout the study. 
 
v

Contents

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

Introduction ....................................................................................................................................................2

Purpose and Scope .............................................................................................................................2

Description of Study Area ..................................................................................................................3

Previous Investigations .......................................................................................................................3

Methods ..........................................................................................................................................................3

Data Collection .....................................................................................................................................5

Continuous Water-Quality Monitoring .....................................................................................5

Discrete Water-Quality Samples ..............................................................................................5

Quality-Assurance and Quality-Control ..................................................................................5

Data Analysis ........................................................................................................................................7

Hydrologic Data ...........................................................................................................................7

Water-Quality Data .....................................................................................................................7

Correlation Analysis ...................................................................................................................7

Hydrology of Lake Houston .........................................................................................................................7

Inflows and Water Residence Time ..................................................................................................7

Relative Thermal Resistance to Mixing ...........................................................................................8

Water Quality in Lake Houston ...................................................................................................................8

Specific Conductance .........................................................................................................................8

pH, Water Temperature, and Dissolved Oxygen ...........................................................................13

Turbidity ...............................................................................................................................................13

Nutrients ..............................................................................................................................................13

Chlorophyll ..........................................................................................................................................14

Taste-and-Odor Causing Organisms and Compounds in Lake Houston ............................................14

Cyanobacteria ....................................................................................................................................15

Actinomycetes Bacteria ...................................................................................................................15

Geosmin and MIB ..............................................................................................................................15

Manganese .........................................................................................................................................17

Microcystin .........................................................................................................................................17

Relations Between Hydrology, Water Quality, and Taste-and-Odor Causing Organisms and 

Compounds in Lake Houston .......................................................................................................17

Summary .......................................................................................................................................................21

References Cited .........................................................................................................................................22

Appendixes 

1. Spearman-rank correlations (r

s

) between environmental variables and taste-andodor causing organisms and compounds at 1-foot depth at Lake Houston Site B in 

the overall dataset, April 2006–September 2008 ..................................................................29

2. Spearman-rank correlations (r

s

) between environmental variables and taste-andodor causing organisms and compounds at 1-foot depth at Lake Houston Site B 

during non-summer (October–May) and summer (June–September), April 2006–

September 2008 ..........................................................................................................................30

3. Spearman-rank correlations (r

s

) between select environmental variables and tasteand-odor causing organisms and compounds at 1-foot depth at Lake Houston 

Site B during short water residence times (less than 100 days), and long water 

residence times (greater than or equal to 100 days), April 2006–September 2008 .........31
vi

Figures

1. Map showing location of monitoring stations in Lake Houston and inflowing 

tributaries and land cover in the San Jacinto River Basin ...................................................4

2–7. Graphs showing: 

2. Daily mean inflow, daily water residence time, and timing of discrete-sample 

collection at Lake Houston Site B during April 2006–September 2008 ......................6

3. Duration curves for daily mean inflow (A) and water residence time (B) at Lake 

Houston during April 2006–September 2008 and relative thermal resistance to 

mixing (C) at Lake Houston Site B during October 2006–September 2008 ................9

4. Duration curves for daily mean specific conductance (A), daily median pH (B), 

daily mean water temperature (C), daily mean dissolved oxygen concentration 

(D), and daily mean turbidity (E) at Lake Houston Site B during October 2006–

September 2008 .................................................................................................................10

5. Daily mean specific conductance, daily mean inflow, and occurrence of 

inflow events resulting in substantial declines in specific conductance at 

Lake Houston Site B during October 2006–September 2008 .....................................11

6. Total cyanobacterial biovolume, biovolume of potential taste-and-odor 

producing cyanobacteria, daily mean water residence time, and daily mean 

water temperature at Lake Houston Site B during April 2006–September 2008 ....16

7. Relations between turbidity and cyanobacterial biovolume (A), turbidity 

and actinomycetes bacteria (B), and actinomycetes bacteria and geosmin 

(C) at Lake Houston Site B during April 2006–September 2008 .................................19

Tables

1. Summary statistics in the overall data set and during non-summer (October–May), 

summer (June–September), short water residence time (less than 100 days), and 

long water residence time (greater than or equal to 100 days) for hydrologic 

characteristics and water-quality constituents measured continuously (hourly) 

and in discrete water-quality samples at 1-foot depth at Lake Houston Site B, April 

2006–September 2008 ...............................................................................................................12

2. Spearman-rank correlations (r

s

) between select environmental variables and 

taste-and-odor causing organisms and compounds at 1-foot depth at Lake Houston 

Site B in the overall dataset and during non-summer (October–May), summer 

(June–September), short water residence time (less than 100 days), and long water 

residence time (greater than or equal to 100 days), April 2006–September 2008 ...........20
vii

Conversion Factors and Datums

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 0.4047 hectare (ha)

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

milliliter (mL) 0.0338 ounces (oz)

acre-foot (acre-ft) 1,233 cubic meter (m3)

cubic micrometer per milliliter 

(μm3/mL)

1.805 x 10-12 cubic inch per ounce (in3/oz)

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Mass

ounce (oz) 28.35 gram (g)

milligram (mg) 0.001 gram (g)

microgram (μg) 0.000001 gram (g)

nanogram (ng) 0.000000001 gram (g)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25°C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).
 
Abstract

Lake Houston is a surface-water-supply reservoir and 

an important recreational resource for the city of Houston, 

Texas. Growing concerns over water quality in Lake Houston 

prompted a detailed assessment of water quality in the reservoir. The assessment focused on water-quality constituents 

that affect the aesthetic quality of drinking water. The hydrologic and water-quality conditions influencing the occurrence of taste-and-odor causing organisms and compounds in 

Lake Houston were assessed using discrete and continuously 

monitored water-quality data collected during April 2006–

September 2008.

The hydrology of Lake Houston is characterized by 

rapidly changing conditions. During inflow events, water residence time can change by orders of magnitude within a matter 

of hours. Likewise, the reservoir can stratify and destratify 

over a period of several hours, even during non-summer and at 

relatively short water residence times, given extended periods 

with warm temperatures and little wind. The rapidly changing 

hydrology likely influences all other aspects of water quality 

in Lake Houston, including the occurrence of taste-and-odor 

causing organisms and compounds.

Water quality in Lake Houston varied with respect to 

season and water residence time but typically was indicative 

of turbid, eutrophic to hypereutrophic conditions. In general, 

turbidity and nutrient concentrations were largest during 

non-summer (October–May) and when water residence times 

were relatively short (less than 100 days), which reflects the 

influence of inflow events on water-quality conditions. Large 

inflow events can cause substantial changes in water-quality 

conditions over relatively short periods of time (hours).

The taste-and-odor causing organisms cyanobacteria and 

actinomycetes bacteria were always present in Lake Houston. 

Cyanobacterial biovolume was largest during summer (June–

September) and when water residence time was greater than 

100 days. Annual maxima in cyanobacterial biovolume 

occurred during July-September of each year, when temperatures were larger than 27 degrees Celsius and water residence 

times were longer than 400 days. In contrast, actinomycetes 

bacteria were most abundant during non-summer and when 

water residence times were less than 100 days, reflecting the 

close association between these organisms and transport of 

suspended sediments.

Geosmin and 2-methylisoborneol are the taste-and-odor 

causing compounds most commonly produced by cyanobacteria and actinomycetes bacteria. Geosmin was detected more 

frequently (62 percent of samples) than 2-methylisoborneol 

(29 percent of samples) in Lake Houston. Geosmin exceeded 

the human detection threshold (10 nanograms per liter) only 

once during the study period and 2-methylisoborneol exceeded 

the human detection threshold twice. Manganese is a naturally 

occurring trace element that can occasionally cause taste-andodor problems in drinking water. Manganese concentrations 

exceeded the human detection threshold (about 50 micrograms 

per liter) in about 50 percent of samples collected near the surface and 84 percent of samples collected near the bottom. The 

cyanotoxin microcystin was detected relatively infrequently 

(16 percent of samples) and at small concentrations (less than 

or equal to 0.2 micrograms per liter).

The abundance of the taste-and-odor causing organisms 

cyanobacteria and actinomycetes bacteria in Lake Houston 

was coupled with inflow events and subsequent changes in 

water-quality conditions. Cyanobacterial biovolume (biomass) 

in Lake Houston was largest during warm periods with little 

inflow and relatively small turbidity values. In contrast, actinomycetes bacteria were most abundant following inflow events 

when turbidity was relatively large. Severe taste-and-odor 

problems were not observed during the study period, precluding quantification of the hydrologic and water-quality conditions associated with large concentrations of taste-and-odor 

causing compounds and development of predictive models. 

Reservoir inflow (water residence time) and turbidity, 

variables related to the abundance of potential taste-andodor causing organisms, are currently (2011) continuously 

measured in Lake Houston, and predictive models could be 

developed in the future when the hydrologic and water-quality 

conditions associated with taste-and-odor problems have been 

better quantified. Seasonal and water residence time influences on water-quality conditions altered relations between 

hydrologic and water-quality conditions and taste-and-odor 

causing organisms and compounds. Future data collection and 

Relations Between Hydrology, Water Quality, and Tasteand-Odor Causing Organisms and Compounds in Lake 

Houston, Texas, April 2006–September 2008

By Amy M. Beussink and Jennifer L. Graham
2 Relations Between Hydrology, Water Quality, and Taste-and-Odor Causing Organisms and Compounds in Lake Houston

development of predictive models need to account for the variability associated with season and water residence time. 

Introduction

Houston, Tex., is the fourth most populous metropolitan 

area in the United States, with an estimated population of 

5.9 million people in 2010 (Texas State Data Center, 2010a). 

Population in the Houston metropolitan area has increased by 

approximately 20 percent over the last decade (2000–10), a 

growth trajectory that is expected to continue for at least 

the next 20 years (Texas State Data Center, 2010b). The city 

of Houston is the regional water provider for the Houston 

metropolitan area and relies on a combination of surfacewater and groundwater supplies for municipal and industrial 

water needs. Water-supply needs will continue to increase 

with ongoing population growth and urban development. 

Currently (2011), about 71 percent of the water supply comes 

from surface-water sources and 29 percent from groundwater sources (City of Houston, 2011). Subsidence, primarily 

because of groundwater withdrawal, has led to regulations to 

reduce groundwater withdrawals in the Houston metropolitan 

area to no more than 20 percent of the total water demand 

(Harris Galveston Subsidence District, 1999). Lake Houston 

is a surface-water-supply reservoir for the city of Houston 

and currently (2011) supplies between 10 to 20 percent of the 

total source-water supply (City of Houston, 2011). In addition 

to providing drinking water, Lake Houston is an important 

recreational resource. Because of future water needs and reliance on surface-water supplies, Lake Houston will become an 

increasingly important source-water supply for the Houston 

metropolitan area. 

Taste-and-odor problems are a concern to drinking-water 

suppliers because of customer dissatisfaction with unpalatable drinking water and increased treatment costs to remove 

taste-and-odor causing compounds. There are many potential 

sources of taste and odor in finished drinking water, including 

biological activity in or chemical contamination of the source 

water, chemicals used during the treatment process, and biological activity or materials present in the distribution system. 

Biological activity associated with naturally occurring algae 

(phytoplankton) in surface source-water supplies is among the 

most common causes of taste-and-odor problems in finished 

drinking water (Taylor and others, 2005).

Geosmin and 2-methylisoborneol (MIB) cause earthy 

and musty tastes and odors and frequently are responsible 

for customer complaints about objectionable drinking water 

because the compounds are detectable by humans at extremely 

small concentrations (10 nanograms per liter). Most tasteand-odor problems associated with geosmin and MIB are 

caused by cyanobacteria (blue-green algae), but bacteria in 

the actinomycetes group also can produce geosmin and MIB 

(Taylor and others, 2005). Cyanobacteria are unique among 

the bacteria because they contain chlorophyll-a, a photopigment used for photosynthesis, and commonly are considered 

part of phytoplankton assemblages rather than bacterial 

assemblages in aquatic ecosystems (Wetzel, 2001). Many 

taste-and-odor producing cyanobacteria also have the potential 

to produce cyanotoxins that can cause illness after exposure 

through drinking water or recreational activities (Chorus and 

Bartram, 1999). The actinomycetes bacteria largely are terrestrial organisms associated with soils; inflow events with large 

suspended-sediment loads might result in taste-and-odor episodes caused by geosmin and MIB produced by actinomycetes 

bacteria (Zaitlin and others, 2003; Zaitlin and Watson, 2006). 

Lake Houston experienced multiple taste-and-odor episodes 

throughout 2005 that were described as earthy and musty and 

believed to be associated with geosmin and MIB produced by 

cyanobacteria (Belhateche and others, 2007).

Since 1983, the U.S. Geological Survey (USGS), in cooperation with the city of Houston, has collected water-quality 

and stage data for Lake Houston, as well as water-quality and 

streamflow data for the major tributaries to Lake Houston (Lee 

and Rast, 1997; Liscum and others, 1999; Sneck-Fahrer and 

others, 2005). Growing concerns over water quality in Lake 

Houston, including the taste-and-odor episodes during 2005, 

prompted a more detailed assessment of water quality in the 

reservoir (Oden and Graham, 2008). The assessment described 

in this report is focused on water-quality constituents that 

affect the aesthetic quality of drinking water such as geosmin, 

MIB, and manganese. Beginning in 2006, a real-time waterquality monitoring network was established in Lake Houston 

to supplement existing real-time water-quality monitors on the 

major tributaries. Discrete water-quality samples also were 

collected and analyzed for nutrients, potential taste-and-odor 

causing organisms and compounds, and other water-quality 

constituents (Beussink and Burnich, 2009). The current (2011) 

study improves the understanding of the hydrologic and waterquality conditions that favor the occurrence of taste-and-odor 

causing organisms and compounds in Lake Houston. 

Purpose and Scope

The purpose of this report is to: (1) describe the hydrology of Lake Houston with respect to inflow, water residence 

time, and thermal stratification, (2) describe the water-quality 

conditions in Lake Houston, including taste-and-odor causing 

organisms and compounds, and (3) assess relations between 

hydrologic and water-quality conditions and taste-and-odor 

causing organisms and compounds in Lake Houston. Data 

collected by the USGS from Lake Houston and inflow from 

tributaries during April 2006–September 2008 were used to 

evaluate the hydrology and water quality of Lake Houston. 

The data analyzed in this report were collected as part of a 

larger water-quality monitoring network in Lake Houston. 

Data from the entire reservoir water-quality monitoring 

network were published for the period April 2006–September 

2008 in Beussink and Burnich (2009). Lake Houston Site B 

is emphasized in this report because of its proximity to the 

Northeast Water Purification Plant (NEWPP) and the influence 

water-quality conditions near the plant have on drinking-water 
Methods 3

treatment processes. Site B also has the longest water-quality 

data record of all the reservoir-monitoring-network sites in 

Lake Houston.

Description of Study Area

Lake Houston, located about 25 miles northeast of 

downtown Houston (fig. 1), is a eutrophic to hypereutrophic 

reservoir and an important source-water-supply reservoir and 

recreational resource for the Houston metropolitan area. The 

dam impounding Lake Houston was constructed on the San 

Jacinto River in 1954 by the city of Houston to provide water 

for public supply and irrigation. The reservoir has a capacity of about 130,000 acre-feet and a surface area of 11,854 

acres; mean depth is about 12 feet (ft) and maximum depth is 

about 50 ft (Liscum and East, 2000; Texas Water Development 

Board, 2004). 

Lake Houston is located at the outlet of the 2,825-square 

mile (mi2) San Jacinto River Basin. The basin at Lake Houston 

can be divided into an eastern subbasin and a western subbasin. The 987-mi2 eastern subbasin represents about 35 

percent of the basin and is rural, with predominantly forest 

and grassland land cover classes (Sneck-Fahrer and others, 

2005) (fig. 1). Tributaries in the eastern subbasin include 

Caney Creek (drains approximately 8 percent of the basin), 

East Fork San Jacinto River (14 percent), Luce Bayou (7 percent), and Peach Creek (5 percent). The 1,756-mi2

 western 

subbasin represents about 62 percent of the basin and is more 

urban (fig. 1), with residential and commercial development, 

major transportation corridors, and fluvial gravel and sand 

mining operations. Tributaries in the western subbasin include 

Cypress Creek (drains approximately 11 percent of the basin), 

Spring Creek (16 percent), and the West Fork San Jacinto 

River (35 percent) (Sneck-Fahrer and others, 2005). Lake 

Houston and the area immediately surrounding the reservoir 

represent the remaining 3 percent of the basin (fig. 1). Land 

use on the Lake Houston shoreline includes forested land, 

residential homes, and a municipal park. Six wastewater-treatment facilities in the immediate vicinity contribute wastewater 

discharge to Lake Houston (Miertschin and Associates, 2007). 

Over 300 municipal wastewater discharges are permitted in 

the basin (Texas Commission on Environmental Quality, 

2011a). 

Previous Investigations

Numerous water-quality studies of Lake Houston have 

been conducted since the mid-1980s, including USGS studies 

in cooperation with the city of Houston and various State and 

university studies. Previous studies of Lake Houston have 

been limited to discrete data; the study described in this report 

includes continuously monitored (hourly) data, which allows 

description of changing water-quality conditions at a much 

finer temporal resolution.

Previous studies indicate that in general, larger loads 

of nutrients, suspended solids, and other water-quality 

constituents are contributed from the western tributaries to 

Lake Houston, largely because of urbanization in the western 

subbasin (Liscum and others, 1999; Van Metre and SneckFahrer, 2002; Sneck-Fahrer and others, 2005). Lake Houston 

displays longitudinal gradients characteristic of reservoirs, 

with larger nutrient concentrations, turbidity, and algal biomass in the riverine zone (up-reservoir) of the reservoir and 

smaller concentrations in the lacustrine zone (down-reservoir) 

near the dam (Conrad, 1986; Sneck-Fahrer and others, 2005). 

Theoretical water residence time in Lake Houston ranges from 

about 12 hours to 400 days; annual mean water residence time 

during 2000–04 ranged from 24 to 176 days (Liscum and East, 

2000; Sneck-Fahrer and others, 2005). Storm runoff generally 

passes through the reservoir within 24 hours. During storm 

events horizontal and vertical mixing is not complete for at 

least one-half the length of the reservoir (Matty and others, 

1987; Kilson, 1992).

Lake Houston is generally a turbid reservoir. SneckFahrer and others (2005) found that the amount of suspended 

sediment in Lake Houston is largely controlled by the amount 

and intensity of rainfall in the watershed and discharge from 

tributary inflows. However, several studies have demonstrated 

that turbidity and suspended sediment also are affected by 

turbulent mixing and resuspension of bottom sediments caused 

by wind (Baca and others, 1982; Matty and others, 1987; 

Sneck-Fahrer and others, 2005). Almost all (77 to 93 percent) 

of the sediment entering Lake Houston is retained in the reservoir (Baca and others, 1982; Matty and others, 1987).

Light attenuation in Lake Houston is caused primarily by 

total suspended solids, including suspended sediment, dissolved organic matter, and suspended organic matter, including phytoplankton (Lee and Rast, 1997). Many studies have 

found that although nutrients are sufficient to cause periodic 

algal blooms, algal growth in Lake Houston usually is limited 

by light because of the highly turbid conditions (Baca and 

others, 1982; Conrad, 1986; Matty and others, 1987; Lee and 

Rast, 1997; Sneck-Fahrer and others, 2005).Water residence 

time and inflow events also are important factors controlling 

algal growth in Lake Houston (Baca and others, 1982; Matty 

and others, 1987). 

Methods

In 2006, the USGS, in cooperation with the city of 

Houston, established a water-quality monitoring network in 

Lake Houston to collect continuous (hourly) data for physicochemical properties (specific conductance, pH, water temperature, turbidity, and dissolved oxygen concentration). Mobile, 

multi-depth continuous-monitoring stations were installed at 

five sites in the southwestern quadrant of Lake Houston where 

reservoir conditions are lacustrine (lake-like). Monitoring 

locations attempted to represent water-quality upstream, near, 
4 Relations Between Hydrology, Water Quality, and Taste-and-Odor Causing Organisms and Compounds in Lake Houston

Figure 1. Location of monitoring stations in Lake Houston and inflowing tributaries and land cover in the San Jacinto River Basin.

#

#

#

#

#

#

#

#

#

WEST FORK 

SAN JACINTO RIVER

SPRING CREEK

CYPRESS CREEK

EAST FORK 

SAN JACINTO

RIVER

PEACH

CREEK

CANEY 

CREEK

LUCE

BAYOU

LAKE

HOUSTON

Conroe

LAKE

CONROE

LAKE

HOUSTON

West Fork

San Jacinto

§ ̈¦45

 ̈¦§45

£¤59

Æ·105

Æ·105

08068500

Spring-Spring

08070200

East Fork

295554095093401

Site B

08069000

Cypress-Westfield

08068090

West Fork

08071280

Luce Bayou

08071000

Peach Creek

08070500

Caney Creek

08072000

Lake Houston

95°30'

95°0'

30°0'

30°30'

30°30'

San Jacinto River Basin

Lake Houston

Houston

TEXA S

San Jacinto 

River Basin

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Land cover classification

High intensity developed

Low intensity developed

Open space developed

Cultivated

Grassland/Shrub

Forest

Woody wetland

Herbaceous wetland

Bare

Open water

Subwatershed boundary

U.S. Geological Survey 

continuous water-quality 

monitoring station and identifier 

08068090 # West Fork

Base from U.S Geological Survey digital data,

Houston-Galveston Area Council (HGAC) 2008 Land Cover

Universal Transverse Mercator Projection, 

Zone 15, North American Datum of 1983
Methods 5

and downstream from the NEWPP. Oden and Graham (2008) 

and Beussink and Burnich (2009) provide a detailed description of the strategy used during 2006–08 to monitor water 

quality in Lake Houston and inflowing tributaries.

The focus of this report is on data collected at reservoir 

monitoring station Site B (Lake Houston at mouth of Jack’s 

Ditch near Houston, Tex., USGS station 295554095093401) 

(fig. 1) because it has the longest data record of all Lake 

Houston monitoring sites (Beussink and Burnich, 2009) and 

is in proximity to the NEWPP. Discrete and continuous data 

collected at Lake Houston Site B between April 1, 2006, and 

September 30, 2008 (discrete data collected from April 1, 

2006, through September 30, 2008, and continuous data collected from October 1, 2006, through September 30, 2008) 

were used to assess relations between hydrologic and waterquality conditions and taste-and-odor causing organisms and 

compounds in Lake Houston.

Data Collection

Continuous Water-Quality Monitoring

Streamflow data from USGS continuous water-quality 

monitoring stations on seven tributaries to the reservoir (fig. 1) 

and reservoir storage (volume) data were used to describe 

reservoir inflows and water residence time during April 2006–

September 2008. Streamflow and reservoir storage data are 

available on the USGS National Water Information System 

website (http://waterdata.usgs.gov/nwis). Streamflow and reservoir storage were measured using standard USGS methods 

(Kenney, 2010; Sauer and Turnipseed, 2010; Turnipseed and 

Sauer, 2010).

Continuous water-quality monitoring at Lake Houston 

Site B began in October 2006 and includes hourly measurements at 1, 6, 12, and 16 ft below the water surface. 

Continuous measurements of specific conductance, pH, water 

temperature, turbidity (YSI 6136 optical turbidity sensor), and 

dissolved oxygen concentration (YSI 6150 optical dissolved 

oxygen sensor) were made by using a YSI 6600EDS waterquality monitor. Monitors were maintained and data were processed in accordance with standard USGS procedures (Wagner 

and others, 2006). Additional detail on continuous waterquality monitoring at Lake Houston is described in Beussink 

and Burnich (2009).

Discrete Water-Quality Samples

Discrete water-quality samples were collected at Lake 

Houston Site B during a range of inflow conditions and water 

residence times from April 2006–September 2008 (fig. 2). 

Water-quality samples were collected at about 1-ft below the 

water surface, at the midpoint depth of the water column, and 

at about 1-ft above the bottom sediments (typically around 16 

ft below the water surface). Physicochemical properties (the 

five continuously monitored properties) were measured at each 

depth where discrete water-quality samples were collected. 

Water-quality samples were analyzed for various properties 

and constituents including nutrients, chlorophyll, taste-andodor causing organisms and compounds, and the cyanotoxin 

microcystin. Geosmin, MIB, and microcystin analyses were 

performed on whole water samples; therefore, the concentrations presented in this report represent the total concentrations 

of these compounds (Graham and others, 2008). Additional 

detail on discrete water-quality sample collection and analysis 

is described in Beussink and Burnich (2009).

Chlorophyll (uncorrected for degradation products) 

was analyzed at the USGS Texas Water Science Center 

laboratory in Shenandoah, Texas. Chlorophyll was extracted 

in heated ethanol (Sartory and Grobbelaar, 1986) and analyzed fluorometrically using U.S. Environmental Protection 

Agency (USEPA) Method 445.0 (Arar and Collins, 1997). All 

discrete water-quality data are available on the USGS National 

Water Information System website (http://waterdata.usgs.gov/

nwis).

Quality-Assurance and Quality-Control

Quality-assurance and quality-control samples were 

collected to evaluate sample-collection procedures and laboratory analyses; quality-assurance and quality-control results 

are summarized in Beussink and Burnich (2009). Chlorophyll 

data are not included in the Beussink and Burnich (2009) 

summary. Laboratory-split replicate samples for chlorophyll 

analysis were collected from all samples. Relative percent 

difference (RPD) was used to evaluate differences between 

replicates. The RPD was calculated using the following 

equation:

RPD A B A B = −  +



 



 



 



 / ×

2

100 (1)

where A and B are concentrations in each replicate pair. Most 

laboratory-split replicate chlorophyll samples (91 percent, 

number of replicate pairs was 23) from Lake Houston Site B at 

1-ft depth had RPDs less than 10 percent (median 3 percent); 

however, one replicate pair had a RPD of 11 percent (collected on January 30, 2008) and one had a RPD of 93 percent 

(collected on May 20, 2008). Laboratory-split replicate samples with large RPDs likely were caused by clumps of algae 

or other chlorophyll-containing plant material that could not 

be homogenized by vigorous shaking. The sample collected 

on January 30, 2008, was retained in data analyses. The replicate with the largest chlorophyll concentration (15.3 micrograms per liter, μg/L) on May 20, 2008, was retained for data 

analysis because it was most similar to chlorophyll concentrations measured in samples collected from 1-ft depths at 

two other Lake Houston sites sampled on that date (23.1 and 

26.8 μg/L).
6 Relations Between Hydrology, Water Quality, and Taste-and-Odor Causing Organisms and Compounds in Lake Houston

Figure 2. Daily mean inflow, daily water residence time, and timing of discrete-sample collection at Lake Houston Site B during April 

2006–September 2008. 

Apr 

May 

June 

July 

Aug 

Sept 

Oct 

Nov 

Dec 

Feb 

Mar 

Apr 

May 

June 

July 

Aug 

Sept 

Oct 

Nov 

Dec 

Feb 

Mar 

Apr 

May 

June 

July 

Aug 

Sept 

Jan 

Jan 

Inflow, in cubic feet per second

10

100

1,000

10,000

100,000

Water residence time, in days

0

100

200

300

400

500

600

700

2006 2007 2008

EXPLANATION

Daily mean inflow

Discrete sample collection

Daily mean water residence time

Annual mean inflow

Long-term mean inflow (1984–2008)

Data Analysis

Hydrologic Data

The seven gaged tributaries to Lake Houston (fig. 1) 

represent all of the major contributing inflows to the reservoir. 

Total daily inflow to the reservoir was calculated as the sum of 

the daily mean streamflow of each of the seven major tributaries. Streamflow has been measured on the seven major tributaries to Lake Houston since 1984; these data were used to 

compare reservoir inflows during April 2006–September 2008 

to historical inflow conditions. Water residence time (WRT) is 

a measure of the average amount of time a water molecule will 

spend in a lake or reservoir; generally, WRT is inversely proportional to inflow. WRT was calculated as the ratio of daily 

mean reservoir volume to daily mean inflow (Kalff, 2002) 

using reservoir storage data and the summed inflow data. 

Relative thermal resistance to mixing (RTRM), a unitless measure of the stability of thermal stratification in a water 

column, represents the relative amount of energy required to 

mix a column of water and is based on temperature-related 

density differences between adjacent depths (Kalff, 2002). 

Large RTRM values occur when temperature-related density 

at a given depth is substantially greater than overlying water, 

indicating a tendency toward thermal stratification. Daily 

RTRM values were calculated for October 2006–September 

2008 using the continuous water-temperature profile data 

in Beussink and Burnich (2009). Data from the 1500 hour 

(3:00 p.m. local time) were selected to calculate daily RTRM 

because the largest vertical gradients in temperature typically 

occurred around that time. Vertical gradients likely were largest in mid-afternoon because the largest water temperatures 

generally lag behind the largest ambient air temperatures and 

incoming solar radiation, which occur at midday. Thermal 
Hydrology of Lake Houston 7

resistance to mixing for the entire water column was calculated by summing the RTRM values from each depth. 

Duration curves were used to describe data for reservoir 

inflow, WRT, and RTRM during the study period. Duration 

curves are cumulative distribution functions and were constructed using hourly values to evaluate frequency and magnitude characteristics. The curves show the percentage of time 

that specified conditions were equaled or exceeded (frequency 

of exceedance; Maidment, 1993). Although several similar 

formulas exist for calculating plotting position, the Weibull 

formula (Helsel and Hirsch, 2002) was used in this study.

Water-Quality Data

Water quality was evaluated using data collected from 

only the 1-ft depth because the water column at Lake Houston 

Site B typically did not thermally stratify, and gradients in 

water-quality conditions generally were small (Beussink 

and Burnich, 2009). Duration curves were used to describe 

continuously measured water-quality data collected during the 

study period using the approach described for hydrologic data. 

Continuous and discrete water-quality data were summarized 

using descriptive statistics for the entire data set, as well as by 

season and water residence time. Seasons were defined as nonsummer (October–May) and summer (June–September), after 

Liscum and others (1999). WRT groups were defined as WRT 

less than 100 days and WRT greater than or equal to 100 days. 

In general, WRTs less than 100 days indicate substantial riverine influence on water-quality conditions, and WRTs greater 

than 100 days indicate substantial influence of in-reservoir 

processes on water-quality conditions (Søballe and Kimmel, 

1987; Kalff, 2002). 

Correlation Analysis

Relations between hydrologic and water-quality 

conditions and taste-and-odor causing organisms and compounds at Lake Houston Site B were described using the 

nonparametric Spearman Rank correlation analysis (Sokal and 

Rohlf, 1995; Helsel and Hirsch, 2002). The Spearman Rank 

correlation analysis tests the strength of the monotonic (consistent increase or decrease) association between two variables 

(X and Y) by ranking the variates and calculating a coefficient 

of rank correlation (rs) scaled to range from minus one to one. 

A positive correlation indicates the tendency of Y to increase 

with increases in X; a negative correlation indicates the tendency of Y to decrease with increases in X. When there is no 

correlation between two variables, rs is near zero. Spearman 

Rank correlation coefficients were considered statistically significant when probability values (p-value) were less than 0.05. 

Spearman Rank correlation analysis assumes that all variables 

are at least ordinal, and the data represent a random sample 

from the population of interest (Hampton, 1994). All analyses 

were conducted using SAS 9.2 software (SAS Institute Inc., 

2009).

Hydrology of Lake Houston

Inflows, WRT, and RTRM are important hydrologic 

characteristics of reservoirs because they affect physical, 

chemical, and biological processes, which affect water quality. 

Streamflow and reservoir storage data were used to describe 

inflows and WRT for Lake Houston during April 2006–

September 2008. Water temperature profile data were used 

to describe RTRM for Lake Houston Site B during October 

2006–September 2008.

Inflows and Water Residence Time

Overall, inflow conditions to Lake Houston were generally representative of years with below-normal to normal 

inflow; annual mean inflows to Lake Houston were 36 percent 

and 18 percent less than the long-term mean (2,000 cubic 

feet per second, ft3/s) during 2006 and 2008, respectively, and 

near-typical (2,080 ft3/s) during 2007 (fig. 2). However, three 

notable inflow events, all more than 25,000 ft3/s, occurred during the study period. The largest inflow to Lake Houston during the study period (58,200 ft3

/s) occurred 2 to 3 days after 

Hurricane Ike made landfall (September 13, 2008). 

Daily mean inflow to Lake Houston during April 2006–

September 2008 ranged from 109 to 58,200 ft3/s and typically 

(80 percent of the time) exceeded 202 ft3/s (fig. 3A, table 1). 

The median of all daily mean inflows during summer (316 

ft3/s) was about 2 times less than the median of all daily mean 

inflows during non-summer (626 ft3/s), though runoff from 

storm events punctuated the hydrograph records during both 

seasons (fig. 2). Increased inflow caused decreased WRT; the 

median of all daily mean inflow was about 8 times larger when 

WRT was less than 100 days (2,070 ft3/s) than when water 

residence time was greater than 100 days (268 ft3/s) (table 1).

Water residence time in Lake Houston during April 

2006–September 2008 ranged from 2 to 647 days and typically (80 percent of the time) exceeded 37 days (fig. 3B, 

table 1). WRT was greater than 100 days (indicating dominance of in-reservoir processes) on 554 days and less than 

100 days (indicating dominance by riverine processes) on 

360 days. Therefore, during the study period, in-reservoir processes likely influenced water-quality conditions more often 

(about 61 percent of the time) than riverine processes. Median 

WRT was about 2 times longer in summer (219 days), when 

inflows were smaller, than in non-summer (114 days) (table 

1), indicating in-reservoir processes likely influenced water 

quality more during summer than non-summer.

Relative Thermal Resistance to Mixing

Relative thermal resistance to mixing at Lake Houston 

Site B ranged from zero (no thermal resistance to mixing, 

indicative of well-mixed conditions) to 296 (indicative of 

thermally stratified conditions). RTRM was less than 56 about 

80 percent of the time (fig. 3C), and the overall median RTRM 
8 Relations Between Hydrology, Water Quality, and Taste-and-Odor Causing Organisms and Compounds in Lake Houston

during the study period was relatively small (17) (table 1), 

indicating that Lake Houston at Site B typically is well mixed. 

RTRM varied by up to an order of magnitude between two 

consecutive days, and extended periods (greater than one 

week) of stable stratification did not occur at Lake Houston 

Site B during October 2006–September 2008. In general, the 

largest RTRM values occurred during summer, when heating of surface water is largest and WRTs are longest. Median 

RTRM in summer (44) was about 4 times larger than in nonsummer (10) (table 1), and median RTRM when WRT was 

greater than 100 days (20) was about 1.5 times larger than 

when WRT was less than 100 days (13). However, RTRM 

ranged from zero to greater than 200 in all seasonal and WRT 

classes, indicating thermal stratification at Lake Houston Site 

B is ephemeral and changes rapidly in response to changing 

hydrological and meteorological conditions. Lake Houston 

Site B is relatively shallow (about 16 ft) compared to the deepest locations in the reservoir (about 50 ft), and RTRM at Site 

B might not be representative of the entire reservoir. However, 

previous studies at deeper locations in Lake Houston also have 

found that stratification rarely occurs (Baca and others, 1982; 

Conrad, 1986; Sneck-Fahrer and others, 2005).

Water Quality in Lake Houston

Water-quality was evaluated using continuous (October 

2006–September 2008) and discrete (April 2006–September 

2008) water-quality data collected at 1-ft depth from 

Lake Houston Site B during April 2006–September 2008. 

Continuous water-quality data included specific conductance, 

pH, water temperature, dissolved oxygen, and turbidity. 

Discrete samples were analyzed for nutrients, chlorophyll, 

taste-and-odor causing organisms and compounds, and the 

cyanotoxin microcystin.

Specific Conductance

Specific conductance is an indirect measure of dissolved 

solids in water. The concentration of dissolved solids in water 

and, therefore, specific conductance is affected by watershed 

characteristics such as soils, geology, and watershed size, as 

well as point-source discharges and urban runoff (Hem, 1992). 

In general, dissolved solids concentrations in rainwater are 

small and inflow events will decrease the specific conductance of a lake or reservoir. Conversely, evaporation of water 

from lakes and reservoirs during periods of little inflow will 

concentrate dissolved solids, increasing specific conductance 

(Kalff, 2002). 

Overall, daily mean specific conductance ranged from 

66.1 to 303 microsiemens per centimeter at 25 degrees Celsius 

(μS/cm) and typically (80 percent of the time) exceeded 

143 μS/cm (fig. 4A). Seasonal differences in specific conductance were not evident. Median specific conductance 

was about 1.2 times larger when WRT was greater than 100 

Figure 3. Duration curves for daily mean inflow (A) and water 

residence time (B) at Lake Houston during April 2006–September 

2008 and relative thermal resistance to mixing (C) at Lake Houston 

Site B during October 2006–September 2008. 

0 80 100

Inflow, in cubic feet per second

10

100

1,000

10,000

100,000 Water residence time, in days

0

100

200

300

400

500

600

700

Frequency of exceedance, in percent

Relative thermal resistance to mixing

0

50

100

150

200

250

300

350

A

B

C

20 40 60

0 8 20 40 60 0 100

0 8 20 40 60 0 100
Table 1. Summary statistics in the overall data set and during non-summer (October–May), summer (June–September), short water residence time long water residence time (greater than or equal to 100 days) for hydrologic characteristics and water-quality constituents measured continuously (hquality samples at 1-foot depth at Lake Houston Site B, April 2006–September 2008.[WRT, water residence time; <, less than; ≥, greater than or equal to; n, number of samples; ft3/s, cubic feet per second; μS/cm, microsiemens per centimeter at 25 degrees Celsius; units; mg/L, milligrams per liter; μg/L, micrograms per liter; ng/L, nanograms per liter; col/100 mL, colonies per 100 milliliters of sample; μm3/L, cubic micrometers per liter; MIB

Hydrologic characteristic or 

water-quality constituent 

(unit of measure)

Season Water Re

All Non-summer Summer WRT < 100 days 

n Range Median n Range Median n Range Median n Range Medi

Hydrologic characteristics

Reservoir inflow1 (ft3/s) 914 109–58,200 487 548 114–35,100 626 366 109–58,200 316 360 678–58,200 2,070 Water residence time2 (days) 914 2–647 144 548 2–608 114 366 2–647 219 360 2–99.2 36.Relative thermal resistance to mixing3 624 0–296 17.0 442 0–205 10.0 182 0–296 44.0 313 0–205 13.

Physicochemical properties

Specific conductance, field4 (μS/cm) 690 66.1–303 186 474 66.1–259 187 216 71.3–303 186 324 66.1–291 166 pH, field4 (standard units) 661 6.5–9.7 7.7 462 6.7–9.7 7.6 199 6.5–9.6 8.2 316 6.5–9.1 7.Water temperature, field4 (degrees Celsius) 699 7.71–32.6 22.3 481 7.71–29.4 18.6 218 24.4–32.6 29.8 325 7.71–31.3 20.Dissolved oxygen, field4 (mg/L) 655 4.0–12 8.1 447 4.0–12 8.5 208 4.2–12 7.4 312 4.2–12 7.Turbidity, field4 (FNU) 619 3.70–180 28.4 440 4.54–180 38.0 179 3.70–83.0 10.3 302 5.10–180 39.

Nutrients

Total nitrogen (mg/L) 29 .67–1.67 .95 13 .81–1.67 1.15 16 .67–1.21 .88 10 .80–1.67 .Total phosphorus (mg/L) 32 .07–.56 .20 14 .11–.56 .24 18 .07–.31 .19 12 .07–.56 .Total nitrogen to total phosphorus ratio 29 2.3–7.3 5.1 13 2.3–7.3 5.5 16 2.6–7.2 4.8 10 2.3–7.2 4.Chlorophyll (μg/L) 23 5.4–61 34 10 5.4–54 16 13 10–61 34 8 8.4–61 37 

Taste-and-Odor Causing Organisms and Compounds

Cyanobacteria (million μm3/mL) 33 .04–2.6 .60 15 .04–.90 .20 18 .09–2.6 1.2 14 .04–2.6 .Percent cyanobacteria5 33 3.3–54 15 15 3.3–28 9.1 18 4.1–54 23 14 3.3–41 14 Potential taste-and-odor producing cyanobacteria (million μm3/mL) 33 .01–1.9 .30 15 .02–.50 .10 18 .01–1.9 .60 14 .01–1.9 .Percent potential taste-and-odor producing 

cyanobacteria6

33 8.9–88 49 15 23–88 45 18 8.9–78 55 14 16–88 46 

Actinomycetes bacteria (col/mL) 26 2–1,900 65 12 43–1,900 140 14 2–160 21 11 6–1,900 160 Geosmin (ng/L) 24 <1.0–14 1.5 13 <1.0–14 1.0 11 <1.0–5.0 2.0 7 <1.0–14 2.MIB (ng/L) 24 <1.0–22 <1.0 13 <1.0–5.0 <1.0 11 <1.0–22 <1.0 7 <1.0–22 <1.

Manganese, 1-foot depth (μg/L) 28 22.0–388 51.5 14 29.0–110 56.0 14 22.0–388 50.0 10 30.0–110 55.

Manganese, near bottom (μg/L) 31 34.0–650 90.0 14 35.0–200 81.0 17 34.0–650 100 11 50.0–170 90.Microcystin (μg/L) 32 <.10–.20 <.10 15 <.10–.10 <.10 17 <.10–.20 <.10 12 <.10–.10 <.

1 Reservoir inflow was calculated as the sum of the daily mean streamflow from the seven major tributaries to Lake Houston for the period April 2006–September 2008.2 Water residence time was calculated for the period April 2006–September 2008 based on daily mean streamflow from the seven major tributaries to Lake Houston and daily m3 Relative thermal resistance to mixing calculations are for the period October 2006–September 2008.4 Continuously monitored properties; summary statistics are based on daily mean values at 1-foot depth, except for pH, which is based on daily median values, for the period O5 Percentage contribution of cyanobacterial biovolume to the total phytoplankton biovolume.6 Percentage contribution of potential taste-and-odor producing cyanobacteria biovolume to the total cyanobacterial biovolume.
10 Relations Between Hydrology, Water Quality, and Taste-and-Odor Causing Organisms and Compounds in Lake Houston

Figure 4. Duration curves for daily mean specific conductance (A), daily median pH (B), daily mean water temperature (C), daily mean 

dissolved oxygen concentration (D), and daily mean turbidity (E) at Lake Houston Site B during October 2006–September 2008. 

A B

C D

E

pH, in standard units Dissolved oxygen, in milligrams per liter

Specific conductance, in microsiemens 

per centimeter at 25 degrees Celsius

Water temperature, in degrees Celsius Turbidity, in formazin nephelometric units

350

300

250

200

150

100

50

40

35

30

25

20

15

0

5

10

200

160

120

100

80

60

0

180

140

20

40

10.0

9.5

9.0

8.5

8.0

7.5

6.0

6.5

7.0

14

12

10

8

6

4

2

Texas Commission on Environmental

Quality minimum standard

Texas Commission on Environmental

Quality minimum standard

Texas Commission on Environmental

Quality maximum standard

Texas Commission on Environmental

Quality maximum standard

Frequency of exceedance, in percent

Frequency of exceedance, in percent

0 20 40 60 80 100

0 20 40 60 80 100

0 20 40 60 80 100

0 20 40 60 80 100 0 20 40 60 80 100
Water Quality in Lake Houston 11

days (208 μS/cm) than when WRT was less than 100 days 

(166 μS/cm) (table 1). In general, extended periods of high 

specific conductance in Lake Houston were associated with 

periods of low inflow and long WRTs, when evaporation leads 

to concentration of dissolved ions. 

In-reservoir responses to inflow events vary depending on storm and rainfall characteristics such as magnitude, 

duration, time since last event, and antecedent watershed and 

tributary conditions. Many inflow events to Lake Houston 

initially caused a 1.1- to 1.5-fold increase in specific conductance (fig. 5), likely because the first flush washed accumulated water-quality constituents into the reservoir (Kilson, 

1992). Following the first flush, specific conductance in Lake 

Houston typically declined. Large inflow events caused 2- to 

3-fold decreases in the specific conductance of Lake Houston 

over relatively short periods of time (hours) (fig. 5), which 

presents challenges to optimizing drinking-water treatment 

processes. When the water quality of raw water changes rapidly, adjustments to drinking-water treatment processes must 

also be made quickly. For example, city of Houston watertreatment plant operators have found that rapid decreases in 

the specific conductance of Lake Houston water are indicative 

of changes in other water-quality properties, which require 

changing to a different type of coagulant or other adjustments 

(Y. Wang and T.L. Lo, City of Houston, written commun., 

2011). During the study period, daily mean specific conductance was less than 75 μS/cm (near the minimum values 

recorded during the study period) less than 2 percent of the 

time (fig. 4A); specific conductance dropped below 75 μS/cm 

in October 2006 and September 2008 when inflows exceeded 

30,000 ft3/s (fig. 5).

Inflows to Lake Houston can be used as an indicator of 

changing water-quality conditions in the reservoir. Increased 

inflows and associated changes in specific conductance for 

select runoff events indicate approximately a 48- to 96-hour 

lead time before large inflows from the tributaries to Lake 

Houston result in changes in specific conductance at Lake 

Houston Site B near the NEWPP drinking-water plant (fig. 5). 

The continuous water-quality monitoring network in Lake 

Houston and the major tributaries to the reservoir might 

provide an advanced warning and allow adjustment to drinking-water treatment processes prior to substantial changes in 

water-quality conditions.

pH, Water Temperature, and Dissolved Oxygen

pH is a measure of the effective hydrogen ion concentration and is used as an index of the status of chemical and 

biological equilibrium reactions in water. The pH of natural 

water generally ranges from 6.5 to 8.5 standard units (Hem, 

1992). Texas water-quality standards for lakes and reservoirs 

require that pH measure not less than 6.5 and not more than 

9.0 standard units (Texas Commission on Environmental 

Quality, 2011b). Daily median pH ranged from 6.5 to 9.7, with 

an overall median of 7.7 (table 1). During the study period, 

daily median pH never dropped below 6.5 and exceeded 9.0 

less than 5 percent of the time (fig. 4B). Exceedances likely 

were associated with periods of high photosynthetic activity. Median daily pH values generally were similar between 

seasons and WRT classes (table 1). 

Water temperature affects water density, the solubility 

of constituents in water, specific conductance, pH, the rate 

Figure 5. Daily mean specific conductance, daily mean inflow, and occurrence of inflow events resulting in substantial declines in 

specific conductance at Lake Houston Site B during October 2006–September 2008. 

Sept

Oct

NovDec

Feb

Mar

Apr

May

June

July

Aug

Sept

Oct

NovDec

Feb

Mar

Apr

May

June

July

Aug

Sept

JanJan

Oct

Specific conductance, in microsiemens 

per centimeter at 25 degrees Celsius

50

100

150

200

250

300

350

Inflow, in cubic feet per second

10

100

1,000

10,000

100,000

1,000,000

2006 2007 2008

EXPLANATION

Daily mean specific conductance

Daily mean inflow

Inflow event
12 Relations Between Hydrology, Water Quality, and Taste-and-Odor Causing Organisms and Compounds in Lake Houston

of chemical reactions, and biological activity in water (Hem, 

1992). Texas water-quality standards for lakes and reservoirs 

require that water temperature not exceed 32.2 degrees Celsius 

(oC) (Texas Commission on Environmental Quality, 2011b). 

Daily mean water temperature ranged from 7.71 to 32.6oC 

with an overall median of 22.3oC (table 1). During the study 

period, daily mean water temperature exceeded the Texas 

Commission on Environmental Quality (TCEQ) standard 

(32.2oC) less than one percent of the time (fig. 4C). Median 

water temperature in summer (29.8oC) was 11.2 degrees 

warmer than in non-summer (18.6oC). When WRT was greater 

than 100 days, median water temperature was 4.4 degrees 

warmer (24.7oC) than when WRT was less than 100 days 

(20.3oC), likely because WRTs typically are longer during 

summer (table 1). 

Dissolved oxygen concentration in surface water is controlled primarily by photosynthetic activity of phytoplankton 

and aquatic plants, atmospheric reaeration, and water temperature (Lewis, 2006). Dissolved oxygen concentration affects 

chemical reactions and the survival of aquatic organisms. 

Texas water-quality standards for lakes and reservoirs require 

that dissolved oxygen concentrations are not less than 5.0 milligrams per liter (mg/L) (Texas Commission on Environmental 

Quality, 2011b). Daily mean dissolved oxygen ranged from 

4.0 to 12 mg/L with an overall median of 8.1 mg/L (table 1). 

During the study period, daily mean dissolved oxygen concentration exceeded 5.0 mg/L 98 percent of the time (fig. 4D). 

Small dissolved oxygen concentrations generally coincided 

with larger water temperatures, which cause decreased oxygen 

solubility in water. The medians of daily mean dissolved 

oxygen concentrations were generally similar between seasons 

and WRT classes (table 1). 

Turbidity

Turbidity, caused by suspended and dissolved matter 

such as clay, silt, fine organic matter, microscopic organisms, organic acids, and dyes (ASTM International, 2003), 

is often used as a surrogate for suspended solids and sediment. Suspended solids include sediment, phytoplankton, 

and other organic and inorganic matter. Increased turbidity 

reduces light penetration and photosynthesis, smothers benthic habitats, and interferes with feeding activities. Relatively 

large turbidity values persisting for short periods might be 

less harmful to benthic invertebrates compared to relatively 

smaller turbidity values persisting for long periods (Wetzel, 

2001). In addition, suspended particulates provide attachment 

sites for nutrients, organic compounds, and other potential 

contaminants. Suspended sediment in surface water contributes to overall turbidity and typically comes from erosion 

and subsequent transport of surface and channel bank soils 

from the watershed and in-lake resuspension of deposited 

sediments (Wetzel, 2001). There are no State water-quality 

standards for turbidity or suspended sediment in Texas lakes 

and reservoirs.

Daily mean turbidity ranged from 3.70 to 180 formazin 

nephelometric units (FNU) with an overall median of 28.4 

FNU (table 1). Turbidity exceeded 10.2 FNU about 80 percent 

of the time during the study period (fig. 4E). Median turbidity during the non-summer (38.0 FNU) was almost 4 times 

larger than during summer (10.3 FNU), likely because of the 

larger inflows and associated transport of sediment and other 

suspended solids during non-summer compared to summer. 

Similarly, median turbidity was about 2 times larger when 

WRT was less than 100 days (39.1 FNU) than when WRT was 

longer (20.1 FNU) (table 1).

Nutrients

Nutrients, particularly nitrogen and phosphorus, are 

considered one of the leading causes of water-quality impairment in the Nation (U.S. Environmental Protection Agency, 

2009). Nutrients are essential for the growth of all organisms; 

however, excessive concentrations in lakes and reservoirs can 

cause nuisance algal growth. Overly abundant algal growth 

causes aesthetic concerns, degrades habitats, and decreases 

dissolved oxygen concentrations (Wetzel, 2001; Kalff, 2002). 

In 2000, the USEPA recommended ecoregion-based 

nutrient criteria for lakes and reservoirs. Lake Houston is 

located in level III, ecoregion 35. The USEPA defined the 

characteristics of water bodies least impacted by human activities (reference conditions) for each ecoregion to describe the 

nutrient concentrations that would protect designated uses 

and mitigate the effects of nutrient enrichment. Reference 

conditions for total nitrogen (TN) and total phosphorus (TP) 

for level III, ecoregion 35 lakes and reservoirs are defined 

as 0.492 and 0.0325 mg/L, respectively; these criteria are 

preliminary and not used for regulatory purposes (U.S. 

Environmental Protection Agency, 2000). TCEQ nutrient 

standards currently (2011) are being developed. Measured 

total nutrient concentrations at Lake Houston Site B during April 2006–September 2008 always exceeded reference 

conditions. TN concentrations (range 0.67 mg/L to 1.67 mg/L; 

median 0.95 mg/L) were about 1.4 to 3.5 times larger than the 

reference conditions and TP concentrations (range 0.07 mg/L 

to 0.56 mg/L; median 0.20 mg/L) were about 2.2 to 17 times 

larger than the reference conditions (table 1). Total nutrient 

data collected from Lake Houston during 1983–90 (Liscum 

and others, 1999) and 2000–04 (Sneck-Fahrer and others, 

2005) show a similar pattern, with TN and TP concentrations 

typically exceeding reference conditions for total nutrients. 

Nutrient concentrations in Lake Houston are typical of those 

found in eutrophic to hypereutrophic lakes and reservoirs 

(Nürnberg, 1996).

Median total nutrient concentrations during non-summer 

(TN median 1.15 mg/L; TP median 0.24 mg/L) (table 1) were 

about 1.3 times larger than during summer (TN median 0.88 

mg/L; TP median 0.19 mg/L), likely because of larger inflows 

and associated transport of nutrients during non-summer. 

Differences in total nutrient concentrations among WRT 
Taste-and-Odor Causing Organisms and Compounds in Lake Houston 13

classes also might reflect the influence of larger inflows, with 

slightly larger median values when WRT was less than 100 

days; however, seasonal differences were more pronounced 

than differences between WRT classes.

Nutrient supply ratios, such as TN to TP (TN:TP), 

reflect the relative amount of each nutrient available for algal 

growth. In general, TN:TP ratios larger than 17 are indicative 

of potential phosphorus limitation, and TN:TP ratios less than 

10 are indicative of potential nitrogen limitation (Forsberg 

and Ryding, 1980). Many cyanobacteria have the ability to 

fix atmospheric nitrogen and can form nuisance blooms when 

nitrogen is limited (small TN:TP ratios) (Wetzel, 2001). At 

Lake Houston Site B, TN:TP ratios were always less than 

8 (median 5.1) during the study period (table 1), indicating 

potential nitrogen limitation and nutrient concentrations favorable for cyanobacterial growth. However, previous studies 

indicate that Lake Houston is likely light-limited, rather than 

nutrient-limited, most of the time (Matty and others, 1987; Lee 

and Rast, 1997; Sneck-Fahrer and others, 2005). 

Chlorophyll

Chlorophyll, a light-gathering pigment present in all 

photosynthetic organisms, often is used as an estimate of algal 

biomass because it is simpler and less time consuming than 

identifying, counting, and measuring algal cells. In 2000, the 

USEPA recommended ecoregion-based chlorophyll criteria for 

lakes and reservoirs. The reference condition for chlorophylla in level III, ecoregion 35 lakes and reservoirs is defined 

as 2.84 micrograms per liter (μg/L) (U.S. Environmental 

Protection Agency, 2000). This criterion was intended as a 

preliminary attempt to describe the chlorophyll-a concentrations that would protect designated uses and is not used for 

regulatory purposes. Chlorophyll criteria have been developed 

for some lakes and reservoirs in Texas but not Lake Houston 

(Texas Commission on Environmental Quality, 2011b).

Chlorophyll concentrations in Lake Houston during April 

2006–September 2008 ranged from 5.4 to 61 μg/L (table 1), 

and always exceeded the reference condition; however, this 

result must be interpreted with caution because the reference 

condition is based only on chlorophyll-a, whereas these data 

represent total chlorophyll concentrations and therefore are 

expected to be larger compared to chlorophyll-a concentrations. The median total chlorophyll concentration (34 μg/L) 

in Lake Houston is indicative of hypereutrophic conditions 

(Nürnberg, 1996).

Median total chlorophyll concentration during summer 

(34 μg/L) was about 2 times larger than during non-summer 

(16 μg/L), likely because of warmer temperatures and longer day length and WRT. Total chlorophyll concentrations 

were similar between the WRT classes; however, they were 

expected to be larger during periods of longer residence 

time because there is more time for growth before algal cells 

are flushed from the system (Søballe and Kimmel, 1987; 

Jørgensen, 2003; Xu and others, 2009). Because water residence time can change so rapidly in Lake Houston during any 

time of the year (fig. 2), seasonal influences (such as temperature and longer day length) might mask the influence of 

WRT on chlorophyll concentration. Sample numbers are too 

small (range 4 to 9) to evaluate chlorophyll by both season and 

WRT. 

Taste-and-Odor Causing Organisms 

and Compounds in Lake Houston

There are many potential sources of taste-and-odor in 

finished drinking water, including biological activity or chemical contamination of the source water, chemicals used in the 

treatment process, and biological activity or materials present 

in the distribution system. Biological activity associated with 

naturally occurring algae in source water is among the most 

common causes of tastes and odors in finished drinking water. 

Geosmin and MIB cause earthy and musty tastes and odors 

and frequently are responsible for customer complaints about 

objectionable drinking water because they are detectable by 

humans at extremely small concentrations (10 nanograms per 

liter, ng/L). Taste-and-odor problems are a concern to drinking-water suppliers because of customer dissatisfaction with 

unpalatable drinking water and increased treatment costs to 

remove taste-and-odor causing compounds (Taylor and others, 

2005). The taste-and-odor causing organisms cyanobacteria 

and actinomycetes bacteria, the taste-and-odor compounds 

geosmin, MIB, and manganese, and the cyanotoxin microcystin were measured in discrete water-quality samples collected 

at Lake Houston Site B during April 2006–September 2008 

(Beussink and Burnich, 2009). 

Cyanobacteria

Biovolume is used to describe the cyanobacterial community in Lake Houston because it is an indicator of biomass 

(Hillebrand and others, 1999). Peak cyanobacterial biovolume 

occurred during summer, with the summer median biovolume 

about 6 times larger than the non-summer median biovolume 

(fig. 6; table 1). Likewise, cyanobacteria contributed about 

2.5 times more to total phytoplankton biovolume during 

summer than non-summer. Larger cyanobacterial biovolume 

during summer likely is because of warmer temperatures and 

longer WRTs; during April 2006–September 2008, annual 

maxima in cyanobacterial biovolume occurred during JulySeptember, when temperatures were greater than 27°C and 

WRTs were longer than 400 days (fig. 6). Although reservoir temperatures and WRTs during summer generally were 

favorable for their growth, cyanobacteria never dominated the 

phytoplankton community (3.3 to 54 percent of total biovolume; median 15 percent) (table 1), and cyanobacterial blooms 

were not observed.

Cyanobacterial taxa that form nuisance blooms, such 

as Anabaena, Aphanizomenon, and Microcystis, are relatively large and typically grow more slowly than other 
14 Relations Between Hydrology, Water Quality, and Taste-and-Odor Causing Organisms and Compounds in Lake Houston

phytoplankton groups (Wetzel, 2001). Therefore, nuisance 

cyanobacteria require longer WRTs to become dominant 

and form blooms and it was expected that cyanobacterial 

biovolume would be larger during periods of long WRT. 

Large declines (7 to 30 fold) in cyanobacterial biovolume 

occurred in response to inflow events of more than 25,000 ft3/s 

(October 2006, January 2007, September 2008) and in general, 

cyanobacterial biovolume was about 1.5 times larger when 

WRTs were greater than 100 days than when WRTs were 

less than 100 days (fig. 6, table 1). As discussed for chlorophyll, seasonal influences might mask the influence of 

WRT on cyanobacterial biovolume. For example, though 

sample numbers are too small for a substantive analysis, 

cyanobacterial biovolume was about 1.7 times larger during 

summer when WRT was greater than 100 days (median 1.7 

million μm3/mL; number of samples [n]=11) than during summer when WRT was less than 100 days (median 1.0 million 

μm3/mL; n=7).

Of the 26 cyanobacterial genera identified in Lake 

Houston (Beussink and Burnich, 2009), eight are known 

potential taste-and-odor producers: Anabaena and 

Aphanizomenon (geosmin producers); Lyngbya, Oscillatoria, 

Planktolyngbya, Pseudanabaena, Synechocococcus (geosmin 

and MIB producers); and Jaaginema (MIB producer). Many 

of these genera also are potential producers of cyanotoxins 

including anatoxin, cylindrospermopsin, microcystin, and 

saxitoxin (Graham and others, 2008). Although potential tasteand-odor and toxin producing taxa can be identified, strain 

isolation and culture are required to conclusively determine 

the organisms that produce these compounds in a given lake 

or reservoir (Taylor and others, 2005). Potential taste-and-odor 

producing cyanobacteria composed 8.9 to 88 percent of the 

total cyanobacterial biovolume (median 49 percent) in Lake 

Houston. Patterns with respect to season and WRT reflected 

the dynamics of the overall cyanobacterial community (fig. 6; 

table 1). 

Actinomycetes Bacteria

Actinomycetes bacteria typically are associated with terrestrial soils and sediment. Abundance of actinomycetes bacteria ranged from 2 to 1,900 colonies per milliliter (col/mL), 

with a median of 65 col/mL (table 1). Median abundance of 

Figure 6. Total cyanobacterial biovolume, biovolume of potential taste-and-odor producing cyanobacteria, daily mean water residence 

time, and daily mean water temperature at Lake Houston Site B during April 2006–September 2008.

Apr

May

June

July

Aug

Sept

Oct

NovDec

Feb

Mar

Apr

May

June

July

Aug

Sept

Jan

Oct

NovDec

Jan

Feb

Mar

Apr

May

June

July

Aug

Sept

10,000

100,000

1,000,000

10,000,000

Cyanobacterial biovolume, in cubic micrometers per milliliter

400

500

600

700

Water residence time, in days

0

100

200

300

20

25

30

35

Water temperature, in degrees Celsius

0

5

10

15

EXPLANATION

Total cyanobacteria

Potential taste-and-ordor producing cyanobacteria

Daily mean water residence time

Daily mean water temperature

2006 2007 2008
Relations Between Hydrology, Water Quality, and Taste-and-Odor Causing Organisms and Compounds in Lake Houston 15

actinomycetes bacteria during non-summer (140 col/mL) 

was about 7 times larger than during summer (21 col/mL), 

likely because of the larger inflows and associated transport of 

sediments during non-summer. Differences in actinomycetes 

abundance among WRT classes also reflected the influence 

of larger inflows; the median concentration was about 4 times 

larger when WRT was less than 100 days (160 col/mL) than 

when WRT was greater than 100 days (43 col/mL).

Geosmin and MIB

Geosmin and MIB are the taste-and-odor causing compounds most commonly produced by cyanobacteria and actinomycetes bacteria. Geosmin was detected more frequently 

(62 percent of samples) than MIB (29 percent of samples) 

at Lake Houston Site B during April 2006–September 2008. 

Concentrations of both compounds were relatively small (typically less than 5 ng/L) throughout the study period (table 1). 

Geosmin exceeded the human detection threshold of 10 ng/L 

once, in January 2007, during the study period, and MIB 

concentrations exceeded the human detection threshold on 

two dates in September 2007. Median geosmin concentration 

was 2 times larger during summer (2 ng/L) than non-summer 

(1 ng/L) (table 1); however, the maximum geosmin concentration occurred during non-summer. Median and maximum 

geosmin concentrations were largest when WRT was less than 

100 days (table 1). Patterns in MIB were difficult to discern 

because so few samples had detectable concentrations of MIB; 

however, maximum concentrations occurred during summer 

when WRT was less than 100 days. Seasonal processes and 

inflow events might influence the occurrence of geosmin and 

MIB in Lake Houston.

Manganese

Manganese is a naturally occurring trace element that 

occasionally can cause taste-and-odor problems in drinking 

water. During periods of thermal stratification, when bottom 

sediments become anoxic, manganese in the bottom sediments 

of lakes and reservoirs can be reduced and dissolved into the 

water column (Hem, 1992); therefore, concentrations in near 

bottom samples also were evaluated as part of this study. 

Larger concentrations of manganese generally occurred in 

water samples collected near the bottom, with the median concentration (90.0 μg/L) about 1.7 times larger than the median 

concentration in samples collected near the surface (51.5 μg/L) 

(table 1). Although seasonal patterns in manganese concentration in surface samples were not evident, in bottom samples 

the median concentration was about 1.2 times larger during 

summer (100 μg/L) compared to the median concentration 

during non-summer (81.0 μg/L), likely because of the periodic 

thermal stratification and subsequent loss of oxygen near the 

sediment-water interface. 

The USEPA secondary drinking-water regulation for 

manganese is 50 μg/L (U.S. Environmental Protection Agency, 

2006), the concentration above which humans can typically 

detect manganese in water. Manganese concentrations measured at Lake Houston Site B exceeded 50 μg/L in 50 percent 

of water samples collected near the surface and 84 percent of 

water samples collected near the bottom. Thus, manganese 

concentrations in untreated water from Lake Houston occasionally might be large enough to cause potential taste-andodor problems in finished drinking water. 

Microcystin

Microcystins are a class of cyanobacterial hepatotoxins common in lakes and reservoirs worldwide. Exposure 

to microcystins can occur through drinking water; however, 

the largest risk of exposure to cyanotoxins is through accidental ingestion and inhalation during recreational activities 

(Chorus and Bartram, 1999). Microcystins currently (2011) 

are on the USEPA drinking-water contaminant candidate list 

and several States include microcystins in their freshwater 

beach monitoring programs (Graham and others, 2009). There 

currently are no regulatory standards for microcystin in the 

United States, but the World Health Organization (WHO) has 

proposed provisional guidelines for exposure to microcystin through finished drinking water (1 μg/L) and recreation 

(20 μg/L) (Chorus and Bartram, 1999). During the study 

period, microcystin was detected in 16 percent of samples. 

All detections were at concentrations less than or equal to 

0.2 μg/L, well below the WHO guidelines (table 1). 

Relations Between Hydrology, Water 

Quality, and Taste-and-Odor Causing 

Organisms and Compounds in Lake 

Houston

Relations between hydrology, water quality, and tasteand-odor causing organisms and compounds were developed 

using correlation analysis to determine the environmental 

conditions that favor occurrence in Lake Houston. Relations 

were developed using the overall data set (all samples at Site 

B, 1-ft depth), as well as the overall data set grouped by season (non-summer and summer), and by WRT (WRT less than 

100 days and WRT greater than or equal to 100 days). Because 

most MIB and microcystin results were non-detections, these 

compounds were not included in the correlation analyses. 

All correlations between environmental variables and tasteand-odor causing organisms and compounds are presented in 

appendixes 1–3.

Chlorophyll is an indicator of algal biomass, and the relations between chlorophyll and environmental variables often 

are used to evaluate the potential factors influencing algal 

growth. Algae need nutrients and light to grow; total phosphorus, total nitrogen, and measures of the light environment 
16 Relations Between Hydrology, Water Quality, and Taste-and-Odor Causing Organisms and Compounds in Lake Houston

(such as turbidity) frequently explain a large percentage of the 

variance in chlorophyll concentrations (Jones and Bachmann, 

1976; Jones and others, 2008). Seasonal differences, such as 

warmer temperatures and longer day lengths during summer, and WRT also can have a substantial influence on algal 

biomass. During April 2006–September 2008, chlorophyll at 

Lake Houston Site B was not significantly correlated with total 

nutrient concentrations (all rs≤ 0.48; all p> 0.07) regardless 

of season or WRT. Likewise, chlorophyll generally was not 

significantly correlated with turbidity during the study period. 

However, when WRT was less than 100 days, chlorophyll was 

significantly and negatively correlated with turbidity (rs=-0.85, 

p<0.01, n=8), indicating both WRT and light might have a 

substantial influence on algal biomass in Lake Houston. Other 

Lake Houston studies have found similar results indicating 

that algal biomass is controlled primarily by washout during 

periods of short WRT and light during periods of longer WRT 

(Baca and others, 1982; Matty and others, 1987).

Cyanobacteria are a natural part of phytoplankton assemblages in lakes and reservoirs and commonly are present in at 

least small abundances. Cyanobacterial biomass and community composition vary seasonally as a result of changes in 

water temperature, solar irradiance, meteorological conditions, 

hydrology, and nutrient supply. In temperate climates, cyanobacteria typically dominate the phytoplankton community during mid-summer to early fall. Conditions favoring cyanobacterial blooms include warmer temperatures, elevated nutrient 

concentrations, small TN:TP ratios, reduced light penetration, 

long WRTs, and a stable water column (Reynolds, 1984; Jones 

and Korth, 1995; Chorus and Bartram, 1999; Wetzel, 2001). 

In general, water-quality conditions at Lake Houston 

Site B (elevated nutrient concentrations, small TN:TP ratios, 

and reduced light penetration) were typical of conditions that 

favor cyanobacteria. However, cyanobacteria did not dominate 

the phytoplankton community, even during summer (table 1), 

which indicates other factors such as severe light limitation, 

might limit cyanobacterial growth. Cyanobacterial biovolume 

was always negatively correlated with turbidity, regardless of 

season or WRT (fig. 7A; table 2). However, the two largest 

turbidity values sampled during the study period corresponded 

with relatively large cyanobacterial biovolumes (fig. 7A). 

These samples with large turbidity values were collected 

during inflow events of more than 25,000 ft3/s (October 2006 

and January 2007) and might not represent the typical relation 

between cyanobacteria and turbidity; for example, cyanobacteria might not have been completely flushed from the system 

at the time of sample collection or might have been flushed to 

Site B from upstream locations. 

Correlations between cyanobacteria and turbidity were 

strongest in the overall data set and when WRT was greater 

than 100 days (both rs=-0.79, p<0.001, n=33). Negative correlations with turbidity indicate that cyanobacterial biovolume was larger when turbidity values were relatively small 

at Lake Houston Site B. Cyanobacterial biovolume also was 

always negatively correlated with total phosphorus and total 

nitrogen, though correlations were not always significant 

(table 2). Correlations between cyanobacterial biovolume and 

total nutrient concentrations typically are positive (Downing 

and others, 2001); the negative correlation in Lake Houston 

likely is indicative of light limitation. A large portion of 

external nutrient loads to reservoirs typically is associated 

with suspended particulates during inflow events (Thornton 

and others, 1990), a pattern reflected in the significant, positive correlations between nutrients and turbidity in Lake 

Houston (TP: rs=0.66, p<0.001, n=32; TN: rs=0.81, p< 0.001, 

n=29). Thus, when nutrient concentrations are large in Lake 

Figure 7. Relations between turbidity and cyanobacterial 

biovolume (A), turbidity and actinomycetes bacteria (B), and 

actinomycetes bacteria and geosmin (C) at Lake Houston Site B 

during April 2006–September 2008.

Turbidity, in formazin nephelometric units

Actinomycetes bacteria, in 

colonies per milliliter

1

10

100

1,000

10,000

Turbidity, in formazin nephelometric units

0 120 140 160

Cyanobacterial biovolume, in cubic 

micrometers per milliliter

10,000

100,000

1,000,000

10,000,000

Actinomycetes bacteria, in colonies per milliliter

10 100 1,000 10,000

Geosmin, in nanograms per liter

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

A

B

C

EXPLANATION

Non-summer, water residence time less than 100 days 

Non-summer, water residence time greater than or equal to 100 days

Summer, water residence time less than 100 days

Summer, water residence time greater than or equal to 100 days

20 40 60 80 100

0 20 40 60 80 100 120 140 160

1
Relations Between Hydrology, Water Quality, and Taste-and-Odor Causing Organisms and Compounds in Lake Houston 17

Table 2. Spearman-rank correlations (r

s

) between select environmental variables and taste-and-odor causing organisms and 

compounds at 1-foot depth at Lake Houston Site B in the overall dataset and during non-summer (October–May), summer (June–

September), short water residence time (less than 100 days), and long water residence time (greater than or equal to 100 days), April 

2006–September 2008.

[Spearman-rank correlation coefficients (rs) shown in bold are significant at p-value less than 0.05; yellow highlight indicates values significant at p-value less 

than 0.001; gray shading indicates the location in the matrix where the variable is correlated with itself (rs=1.0); <, less than; ≥, greater than or equal to]

Environmental variables Taste-and-odor causing organisms and compounds

Cyanobacteria Actinomycetes bacteria Geosmin

All data

Water residence time 0.40 -0.59 -0.17

Relative thermal resistance to mixing -.09 -.21 -.06

Turbidity -.79 .88 .08

Total nitrogen -.63 .71 .03

Total phosphorus -.51 .68 .19

Geosmin .09 .13

Non-summer

Water residence time .19 -.36 -.59

Relative thermal resistance to mixing .07 -.17 .07

Turbidity -.48 .80 .38

Total nitrogen -.37 .32 .36

Total phosphorus -.19 .63 .38

Geosmin .10 .36

Summer

Water residence time .51 -.66 .25

Relative thermal resistance to mixing -.52 .00 -.17

Turbidity -.64 .58 -.14

Total nitrogen -.33 .23 -.38

Total phosphorus -.22 .12 -.11

Geosmin -.23 -.01

Water residence time < 100 days

Water residence time .16 -.51 -.04

Relative thermal resistance to mixing -.21 -.10 -.34

Turbidity -.74 .81 .34

Total nitrogen -.61 .50 .43

Total phosphorus -.64 .78 .41

Geosmin -.32 .94

Water residence time ≥ 100 days

Water residence time .58 -.62 -.26

Relative thermal resistance to mixing -.30 -.11 .21

Turbidity -.79 .80 -.01

Total nitrogen -.63 .74 -.07

Total phosphorus -.32 .46 .09

Geosmin .14 -.11
18 Relations Between Hydrology, Water Quality, and Taste-and-Odor Causing Organisms and Compounds in Lake Houston

Houston, turbidity also is large and likely limits algal growth. 

Cyanobacterial biovolume was always positively correlated 

with WRT; the strongest correlations were during summer 

(rs=0.51, p=0.03, n=18 and when WRTs were already greater 

than 100 days (rs=0.58, p<0.01, n=19), indicating periodic 

inflow events might be important in controlling cyanobacterial biovolume during times when it would likely dominate 

otherwise.

Another factor potentially limiting cyanobacterial growth 

and bloom formation in Lake Houston is the well-mixed 

water column (small RTRM). Dense cyanobacterial blooms 

occur most often during extended periods of long WRT and 

stable stratification (large RTRM) (Reynolds, 1984; Jones 

and Korth, 1995; Chorus and Bartram, 1999; Wetzel, 2001). 

Cyanobacterial biovolume was not significantly correlated 

with RTRM in Lake Houston (table 2), likely because RTRM 

varied on a daily basis and Lake Houston Site B did not stably 

stratify for extended periods of time during the study period.

The abundance of actinomycetes bacteria was always 

significantly and positively correlated with turbidity and 

negatively related to WRTs in Lake Houston (fig. 7B, table 2), 

reflecting the tendency of these organisms to be associated 

with large suspended-sediment loads during inflow events 

(Zaitlin and others, 2003; Zaitlin and Watson, 2006). The 

correlation between actinomycetes bacteria and turbidity was 

strongest in the overall data set (rs=0.88, p<0.001, n=26) and 

weakest (rs=0.58, p=0.03, n=14) during summer, when WRTs 

typically are longer. However, the abundance of actinomycetes 

bacteria was most strongly correlated with WRT during summer (rs

=-0.66, p, p=0.01, n=14), indicating inflows likely are 

still influencing actinomycetes abundance. These correlations 

indicate that actinomycetes-related taste-and-odor events in 

Lake Houston are most likely to occur during runoff events or 

wind events that resuspended reservoir bottom sediments.

Most taste-and-odor problems associated with geosmin 

and MIB are caused by cyanobacteria (Taylor and others, 

2005), but relations between water-quality conditions, such as 

nutrients and light, and cyanobacterial biovolume (biomass), 

community composition, and production of nuisance compounds are complex (Graham and others, 2004; Christensen 

and others, 2006; Dzialowski and others, 2009), and further 

complicated by extreme hydrologic variability (Vanni and 

others, 2006). In the overall data set, geosmin was not significantly correlated with any measured variables, including cyanobacterial biovolume (table 2; appendix 1). Correlations with 

the biovolume of only those cyanobacteria that can produce 

taste-and-odor causing compounds and individual cyanobacterial taxa also were not significant (all p>0.05). 

Geosmin and MIB also can be produced by actinomycetes bacteria, though they are less frequently linked to tasteand-odor problems and are not as well studied as the cyanobacteria (Zaitlin and others, 2003; Taylor and others, 2005; 

Zaitlin and others, 2006). Noteworthy was the strong, positive 

correlation between actinomycetes bacteria and geosmin 

when water residence time was less than 100 days (rs=0.94, 

p=0.005, n=6) (fig. 7C, table 2). This result indicates that 

actinomycetes bacteria might occasionally be linked to tasteand-odor problems in Lake Houston. However, this result must 

be interpreted with caution because the number of samples in 

this analysis (n=6) was small. Water residence time was the 

only other variable significantly correlated with geosmin. The 

negative correlation between geosmin and WRT during nonsummer (rs

=-0.59, p=0.03, n=13) indicates geosmin concentrations are larger during periods of shorter WRT. The correlation between geosmin and WRT might reflect the greater 

abundance of actinomycetes bacteria during periods of shorter 

WRT, though the correlation between actinomycetes bacteria 

and geosmin was not significant in non-summer (table 2).

Manganese concentrations can occasionally be large 

enough to cause taste-and-odor problems in Lake Houston, 

especially during periods when stratification and subsequent 

loss of oxygen near the sediment-water interface results in 

release of manganese from the sediments. However, manganese was weakly, and negatively, correlated with RTRM 

(stratification) (rs=-0.49, p=0.03, n=19), indicating manganese concentrations decreased as the strength of stratification 

increased. The negative correlation might reflect a decrease 

in manganese concentration near the surface because of algal 

uptake or sedimentation losses. Because manganese concentrations typically were larger near the bottom, the correlation 

with RTRM also was evaluated using bottom data; the correlation was similar to 1-ft depth, though not statistically significant (rs

=-0.49, p=0.08, n=20).

The hydrology of Lake Houston is characterized by 

rapidly changing conditions. During inflow events WRT 

can change by orders of magnitude within a matter of hours. 

Likewise, the reservoir can stratify and destratify over a period 

of several hours, even during non-summer and at relatively 

short WRTs, given extended periods with warm temperatures 

and little wind. The rapidly changing hydrology likely influences all other aspects of water quality, including the occurrence of taste-and-odor causing organisms and compounds, in 

Lake Houston. 

The abundance of the taste-and-odor causing organisms 

cyanobacteria and actinomycetes bacteria in Lake Houston 

were coupled with inflow events and subsequent changes in 

water-quality conditions. Cyanobacterial biovolume was largest during warm periods with little inflow and relatively small 

turbidity values. In contrast, actinomycetes bacteria were 

most abundant following inflow events when turbidity values 

were relatively large. Because severe taste-and-odor problems 

were not observed during the study period, quantification of 

the hydrologic and water-quality conditions associated with 

large concentrations of taste-and-odor causing compounds 

and development of predictive models were not possible. 

Reservoir inflow (WRT) and turbidity, variables related to 

the abundance of potential taste-and-odor causing organisms, 

are currently (2011) continuously measured in Lake Houston, 

and predictive models could be developed in the future when 

the hydrologic and water-quality conditions associated with 

taste-and-odor problems have been better quantified. Seasonal 

and WRT influences on water-quality conditions altered the 
Summary 19

relations between hydrologic and water-quality conditions 

and taste-and-odor causing organisms and compounds. Future 

data collection and development of predictive models need to 

account for the variability associated with season and WRT. 

Summary

Lake Houston is a surface-water-supply reservoir and 

an important recreational resource for the city of Houston, 

Texas. Growing concerns over water quality in Lake Houston 

prompted a detailed assessment of water quality in the reservoir. The assessment focused on water-quality constituents 

that affect the aesthetic quality of drinking water. Beginning 

in 2006, a real-time water-quality-monitoring network was 

established on Lake Houston to supplement existing realtime water-quality monitors on the major tributaries. Discrete 

water-quality samples also were collected and analyzed for 

nutrients, potential taste-and-odor causing organisms and 

compounds, and other water-quality constituents. The purpose 

of this report is to describe the hydrologic and water-quality 

conditions influencing the occurrence of taste-and-odor causing organisms and compounds from April 1, 2006, through 

September 30, 2008, at Lake Houston Site B, located in proximity to the Northeast Water Purification Plant. This report 

includes: (1) description of the hydrology of Lake Houston 

with respect to inflow, water residence time, and thermal 

stratification; (2) description of the water-quality conditions 

in Lake Houston, including taste-and-odor causing organisms 

and compounds; and (3) assessment of the relations between 

hydrologic and water-quality conditions and taste-and-odor 

causing organisms and compounds in Lake Houston. This 

information improves the understanding of the hydrologic and 

water-quality conditions that favor the occurrence of taste-andodor causing organisms and compounds in Lake Houston.

Streamflow data from gages on seven tributaries to Lake 

Houston and reservoir storage data were used to describe 

reservoir inflows and water residence time during April 

2006–September 2008. A continuous water-quality monitor 

was operated at Lake Houston Site B during October 2006–

September 2008 and measured physicochemical water-quality 

properties, including specific conductance, pH, water temperature, turbidity, and dissolved oxygen at 1, 6, 12, and 16 

feet below the water surface. Relative thermal resistance to 

mixing, a measure of the stability of thermal stratification 

in the water column, was calculated for the period October 

2006–September 2008 using the continuous water-temperature profile data from the 1500 hour (3:00 p.m. local time). 

Discrete water-quality samples were collected at about 1 foot 

below the water surface, at the midpoint depth of the water 

column, and at about 1 foot above the bottom during April 

2006–September 2008 and analyzed for physicochemical 

properties, nutrients, chlorophyll, the taste-and-odor causing 

organisms cyanobacteria and actinomycetes bacteria, the tasteand-odor causing compounds geosmin, 2-methylisoborneol 

(MIB), and manganese, and the cyanotoxin microcystin. Water 

quality was evaluated using data collected from only the 1-foot 

depth because Lake Houston Site B typically did not thermally 

stratify, and gradients in water-quality conditions generally 

were small.

Inflows to Lake Houston were about 2 times less during 

summer (June–September) than non-summer (October–May), 

though storm events punctuated the hydrograph during both 

seasons. Water residence time was about 2 times longer during 

summer, when inflows were smaller, than in non-summer, 

indicating in-reservoir processes likely influenced water quality more during summer than non-summer. Thermal stratification at Lake Houston Site B occurred more frequently during 

summer than non-summer, but stratification was ephemeral 

and changed rapidly in response to changing hydrological and 

meteorological conditions.

Large inflow events can cause substantial changes in 

water-quality conditions, such as specific conductance, over 

relatively short periods of time (hours). There was approximately a 48- to 96-hour lead time before large inflow events 

detected in the tributaries resulted in changes in water-quality 

conditions at Lake Houston Site B. The continuous waterquality monitoring network in Lake Houston and the major 

tributaries to the reservoir might provide an advanced warning 

and allow adjustment to drinking-water treatment processes 

prior to substantial changes in water-quality conditions.

Water quality at Lake Houston Site B varied with respect 

to season and water residence time but typically was indicative 

of turbid, eutrophic to hypertrophic conditions. Total nitrogen 

to total phosphorus ratios were always less than 8 during the 

study period, indicating potential nitrogen limitation and nutrient conditions favorable for cyanobacterial growth. In general, 

turbidity and nutrient concentrations were largest during nonsummer, and when water residence times were relatively short 

(less than 100 days), reflecting the influence of inflow events 

on water-quality conditions. Chlorophyll concentrations, an 

indicator of algal biomass, were about 2 times larger during 

summer than non-summer, likely because of warmer temperatures and longer day lengths and water residence times. 

Cyanobacteria and actinomycetes bacteria were always 

present at Lake Houston Site B. Cyanobacterial biovolume 

(biomass) was about 6 times larger during summer than nonsummer and 1.5 times larger when water residence time was 

greater than 100 days than when water residence time was less 

than 100 days. Annual maxima in cyanobacterial biovolume 

occurred during July-September each year, when temperatures 

were larger than 27 degrees Celsius and water residence times 

were longer than 400 days. In contrast, actinomycetes bacteria 

were about 7 times more abundant during non-summer than 

summer, and 4 times more abundant when water residence 

times were relatively short (less than 100 days), reflecting the 

close association between these organisms and transport of 

suspended sediments by large inflows.

Geosmin and MIB are the taste-and-odor causing compounds most commonly produced by cyanobacteria and actinomycetes bacteria. Geosmin was detected more frequently 

(62 percent of samples) than MIB (29 percent of samples) at 
20 Relations Between Hydrology, Water Quality, and Taste-and-Odor Causing Organisms and Compounds in Lake Houston

Lake Houston Site B. Geosmin exceeded the human detection threshold (10 nanograms per liter) only once during the 

study period and MIB exceeded the human detection threshold 

twice. Manganese is a naturally occurring trace element that 

can occasionally cause taste-and-odor problems in drinking 

water. Because manganese in bottom sediments of lakes and 

reservoirs can be reduced and dissolved into the water column 

during periods of thermal stratification when bottom sediments 

become anoxic, manganese concentrations in bottom samples 

also were evaluated as part of this study. Manganese concentrations exceeded the human detection threshold (about 50 

micrograms per liter) in about 50 percent of samples collected 

near the surface and 84 percent of samples collected near the 

bottom. The cyanotoxin microcystin was detected relatively 

infrequently (16 percent of samples) and at small concentrations (less than or equal to 0.2 micrograms per liter).

Cyanobacterial biovolume in Lake Houston was always 

negatively correlated with turbidity and total nutrient concentrations, relations likely indicative of light limitation of 

cyanobacterial growth. Cyanobacterial biovolume was always 

positively correlated with water residence time, but correlations were strongest during summer and when water residence 

times were already greater than 100 days, indicating periodic 

inflow events might be important in controlling cyanobacterial 

biomass during times when it would likely dominate otherwise. The abundance of actinomycetes bacteria was always 

significantly and positively correlated with turbidity and negatively related to water residence times, reflecting the tendency 

of these organisms to be associated with large suspendedsediment loads during inflow events. In the overall data set, 

geosmin was not significantly correlated with any measured 

variables at Lake Houston Site B. There was a strong, positive 

correlation between actinomycetes bacteria and geosmin when 

water residence time was less than 100 days; however, this 

result must be interpreted with caution because the number of 

samples in this analysis was small.

The abundance of the taste-and-odor causing organisms 

cyanobacteria and actinomycetes bacteria in Lake Houston 

was coupled with inflow events and subsequent changes in 

water-quality conditions. Cyanobacterial biovolume (biomass) was largest during warm periods with little inflow and 

relatively small turbidity. In contrast, actinomycetes bacteria 

were most abundant following inflow events when turbidity 

was relatively large. However, severe taste-and-odor problems were not observed during the study period, precluding 

quantification of the hydrologic and water-quality conditions 

associated with large concentrations of taste-and-odor causing 

compounds and development of predictive models. Reservoir 

inflow (water residence time) and turbidity, variables related 

to the abundance of potential taste-and-odor causing organisms, are currently (2011) continuously measured in Lake 

Houston. Predictive models could be developed in the future 

when the hydrologic and water-quality conditions associated 

with taste-and-odor problems have been better quantified. 

Seasonal and water residence time influences on waterquality conditions altered relations between hydrologic and 

water-quality conditions and taste-and-odor causing organisms 

and compounds. Future data collection and development of 

predictive models need to account for the variability associated 

with season and water residence time. 

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Appendixes
 
Appendix 1 25

Appendix 1

Appendix 1. Spearman-rank correlations (r

s

) between environmental variables and taste-and-odor causing organisms and compounds 

at 1-foot depth at Lake Houston Site B in the overall dataset, April 2006–September 2008. 

[Spearman-rank correlation coefficients (rs) shown in bold are significant at p-value less than 0.05; n, number of samples; yellow highlight indicates values 

significant at p-value less than 0.001; gray shading indicates the location in the matrix where the variable is correlated with itself (rs=1.0); --, value presented in 

upper part of the matrix] 

Hydrologic characteristic or 

water-quality constituent

Taste-and-odor causing organisms and compounds

Cyanobacteria Actinomycetes 

bacteria Geosmin Manganese

rs n rs n r

s n rs n

Water residence time 0.40 33 -0.59 26 -0.17 24 0.18 28

Relative thermal resistance to mixing -.09 23 -.21 22 -.06 16 -.49 19

Specific conductance .51 33 -.63 26 -.15 24 .52 28

pH .58 33 -.82 26 -.16 24 .09 28

Water temperature .61 33 -.76 26 -.04 24 .05 28

Dissolved oxygen -.05 32 -.13 25 -.01 23 -.16 27

Turbidity -.79 33 .88 26 .08 24 -.21 28

Total nitrogen -.63 27 .71 22 .03 24 -.26 27

Total phosphorus -.51 30 .68 23 .19 24 -.36 28

Chlorophyll .23 22 -.28 22 -.55 18 -.12 21

Cyanobacteria -.75 26 .09 23 .43 27

Actinomycetes bacteria -- -- .13 19 -.35 22

Geosmin -- -- -- -- -.12 24

Manganese -- -- -- -- -- --
26 Relations Between Hydrology, Water Quality, and Taste-and-Odor Causing Organisms and Compounds in Lake Houston

Appendix 2

Appendix 2. Spearman-rank correlations (r

s

) between environmental variables and taste-and-odor causing organisms and compounds 

at 1-foot depth at Lake Houston Site B during non-summer (October–May) and summer (June–September), April 2006–September 2008. 

[Spearman-rank correlation coefficients (rs) shown in bold are significant at p-value less than 0.05; n, number of samples; yellow highlight indicates values 

significant at p-value less than 0.001; gray shading indicates the location in the matrix where the variable is correlated with itself (rs=1.0); --, value presented in 

upper part of the matrix] 

Hydrologic characteristic or 

water-quality constituent

Taste-and-odor causing organisms and compounds

Cyanobacteria Actinomycetes 

bacteria Geosmin Manganese

r

s n rs n rs n rs n

Non-summer

Water residence time 0.19 15 -0.36 12 -0.59 13 0.27 14

Relative thermal resistance to mixing .07 12 -.17 11 .07 9 -.55 10

Specific conductance .55 15 -.74 12 -.27 13 .31 14

pH .64 15 -.86 12 -.19 13 .19 14

Water temperature .24 15 -.35 12 -.42 13 .34 14

Dissolved oxygen .28 14 -.67 11 .28 12 -.17 13

Turbidity -.48 15 .80 12 .38 13 -.18 14

Total nitrogen -.37 12 .32 9 .36 13 -.56 13

Total phosphorus -.19 13 .63 10 .38 13 -.34 14

Chlorophyll .07 10 -.36 10 -.49 9 .01 10

Cyanobacteria -.40 12 .10 12 .46 13

Actinomycetes bacteria -- -- .36 9 -.09 10

Geosmin -- -- -- -- -.45 13

Manganese -- -- -- -- -- --

Summer

Water residence time .51 18 -.66 14 .25 11 .16 14

Relative thermal resistance to mixing -.52 11 .00 11 -.17 7 -.47 9

Specific conductance .86 18 -.86 14 .08 11 .71 14

pH .43 18 -.69 14 -.28 11 .05 14

Water temperature .17 18 -.53 14 -.06 11 .14 14

Dissolved oxygen .09 18 -.45 14 -.07 11 -.16 14

Turbidity -.64 18 .58 14 -.14 11 -.31 14

Total nitrogen -.33 15 .23 13 -.38 11 -.34 14

Total phosphorus -.22 17 .12 13 -.11 11 -.37 14

Chlorophyll .08 12 .12 12 -.69 9 -.15 11

Cyanobacteria -.72 14 -.23 11 .66 14

Actinomycetes bacteria -- -- -.01 10 -.73 12

Geosmin -- -- -- -- .29 11

Manganese -- -- -- -- -- --
Appendix 3 27

Appendix 3

Appendix 3. Spearman-rank correlations (r

s

) between select environmental variables and taste-and-odor causing organisms and 

compounds at 1-foot depth at Lake Houston Site B during short water residence times (less than 100 days), and long water residence 

times (greater than or equal to 100 days), April 2006–September 2008. 

[Spearman-rank correlation coefficients (rs) shown in bold are significant at p-value less than 0.05; n, number of samples; yellow highlight indicates values 

significant at p-value less than 0.001; gray shading indicates the location in the matrix where the variable is correlated with itself (rs=1.0); --, value presented in 

upper part of the matrix] 

Hydrologic characteristic or 

water-quality constituent

Taste-and-odor causing organisms and compounds

Cyanobacteria Actinomycetes 

bacteria Geosmin Manganese

rs n rs n r

s n rs n

Water residence time < 100 days

Water residence time 0.16 14 -0.51 11 -0.04 7 -0.50 9

Relative thermal resistance to mixing -.21 12 -.10 11 -.34 7 -.53 10

Specific conductance .56 14 -.50 11 -.22 7 .18 10

pH .57 14 -.60 11 -.11 7 -.07 10

Water temperature .60 14 -.62 11 -.51 7 .07 10

Dissolved oxygen .14 13 -.21 10 .68 6 -.35 9

Turbidity -.74 14 .81 11 .34 7 .08 10

Total nitrogen -.61 9 .50 7 .43 7 -.20 9

Total phosphorus -.64 11 .78 8 .41 7 -.14 10

Chlorophyll .67 8 -.88 8 -.70 6 .19 8

Cyanobacteria -.61 11 -.32 7 .58 10

Actinomycetes bacteria -- -- .94 6 -.36 8

Geosmin -- -- -- -- -.37 7

Manganese -- -- -- -- -- --

Water residence time ≥ 100 days

Water residence time .58 19 -.62 15 -.26 17 .57 18

Relative thermal resistance to mixing -.30 11 -.11 11 .21 9 -.43 10

Specific conductance .43 19 -.70 15 -.08 17 .69 18

pH .54 19 -.83 15 -.16 17 .11 18

Water temperature .53 19 -.83 15 .09 17 .06 18

Dissolved oxygen -.38 19 .06 15 -.17 17 -.20 18

Turbidity -.79 19 .80 15 -.01 17 -.30 18

Total nitrogen -.63 18 .74 15 -.07 17 -.36 18

Total phosphorus -.32 19 .46 15 .09 17 -.45 18

Chlorophyll .02 14 .00 15 -.51 12 -.35 13

Cyanobacteria -.73 15 .14 16 .36 17

Actinomycetes bacteria -- -- -.11 13 -.38 14

Geosmin -- -- -- -- -.17 17

Manganese -- -- -- -- -- --

Publishing support provided by

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Beussink and Graham—Relations Between Hydrology, Water Quality, and Taste-and-Odor Causing Organisms and Compounds in Lake Houston—SIR 2011–5121

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