Draft Appendix G – Chesapeake Bay TMDL for Nutrients and Sediment

G-1 September 24, 2010

Appendix G. Critical Period Analysis

This appendix is a deliberative document related to determination of an appropriate critical 

period for developing the Chesapeake Bay total maximum daily load (TMDL).
Draft Appendix G – Chesapeake Bay TMDL for Nutrients and Sediment

G-2 September 24, 2010

Appendix G - Determination of Critical Conditions for the Chesapeake Bay 

TMDL

Introduction

The Chesapeake Bay TMDL must be developed to attain applicable water quality standards. 

Critical conditions for stream flow pollutant loading and water quality parameters must be taken 

into account. The objective is to select a three-year period as the critical period. The Water 

Quality Goal Team decided that the critical period would be selected from the previously 

selected hydrologic period 1991-2000 because that timeframe is representative of long-term 

hydrology, is within the model calibration period and would facilitate modeling operations (see 

Section 6.1.1 and Appendix F). A three-year period was selected in order to coincide with the 

Chesapeake Bay water quality criteria assessment period (USEPA 2003). The critical period 

should be representative of an approximate 10-year return period to maintain consistency with 

other TMDLs develop and published by the Chesapeake Bay watershed jurisdictions. 

The following sections discuss the process for determining the critical period based on 

determining the return period for each of the three-year time frames within the 1991-2000 

hydrologic period using various methods. 

Approaches Used in Previous TMDLs to Select the Critical Period

To determine if there is consistent approach to establishing a critical period among the 

Chesapeake Bay watershed jurisdictions, Tetra Tech staff explored each jurisdiction’s water 

quality standards, polled the seven watershed jurisdictions, and referenced previously completed 

TMDLs.

Generally, the jurisdictions’ water quality standards do not address a method for establishing the 

critical hydrologic period. Further, EPA does not have specific guidance or regulations on how

to determine critical period. EPA only requires that critical conditions and seasonal variations are 

considered (40 CFR §130.7 (c)(1)). EPA Region 3’s approach has been that jurisdictions may 

use any method for determining critical conditions and seasonal variations as long as the 

approach is supported by sound science. 

In polling the jurisdictions regarding their approaches to determining the hydrology critical 

period, all jurisdictions reported that the determination is dependent on the pollutant, the water 

quality standards, the TMDL endpoint and the amount of flow data available. All jurisdictions 

reported that the critical period was determined using a representative data set capturing a range 

of high, low and average flows. Maryland, the District of Columbia and Virginia reported 

selecting the critical period based on using a dry year, an average year and a wet year. Maryland 

also indicated that in some TMDLs time-variable models use the worst condition in the 

calibration period. Although, nutrient TMDLs with steady state models use 7Q10 flows as the 

critical period. Delaware reported using the 7Q10 for free flowing streams and using the 

monthly or seasonally average as the critical condition for the calibration period for tidal 

streams. Pennsylvania reported recently beginning to use the growing season average as the 

critical period for nutrient TMDLs. West Virginia watershed TMDLs use representative 

precipitation induced flow data over a 6-year period with high, low and average conditions. 

A review of TMDLs completed for tidal influenced streams and estuaries along the Atlantic and 

Gulf Coasts revealed that there is no consistent method for determining the critical period. This 
Draft Appendix G – Chesapeake Bay TMDL for Nutrients and Sediment

G-3 September 24, 2010

review was not intended to be exhaustive, but to reveal general patterns of methodology across 

the country. Most TMDLs used a critical period that was protective during low flows, rather 

than high flows, which is the condition of interest for the Chesapeake Bay TMDL. 

The most commonly identified method for establishing the critical period was the use of 7Q10 

flows. The Louisiana Standard Operating Procedures for Louisiana TMDL Technical 

Procedures (LDEQ 2009) specifically outlines the summer critical conditions as 7Q10 or 0.1 cfs, 

whichever is greater, or for tidal streams 1/3 of the average or typical flow averaged over one 

tidal cycle. Similarly winter critical conditions are 7Q10 of 1 cfs, whichever is greater, or for 

tidal streams 1/3 of the average or typical flow averaged over one tidal cycle. 

Other examples of using 7Q10 flows include:

Total Maximum Daily Load Analysis for Nanticoke River and Broad Creek, Delaware 

(DNREC 1998) The model for this dissolved oxygen (DO), total nitrogen, and total phosphorus 

TMDL was developed and calibrated using hydrologic and hydrodynamic from 1992, a dry year. 

Hydrodynamic Model was run using 7Q10 flows, water quality model was run using 1992 

pollutant loads.

Organic Enrichment/Dissolved Oxygen TMDL Rabbit Creek and Dog River,

Alabama (ADEM 2005). The hydrology of the LSPC model was calibrated for the 

period of record, October 1, 1996 through September 30, 2000. For the purposes of this 

TMDL the 2000-year was utilized as the critical low flow period. 2000 was a relatively 

dry year and was one of the time periods over which the models were calibrated, lending 

confidence to the simulations. The time period of the model simulation was from 2000 to 

2001. This time period was selected based on the availability and relevance of the 

observed data to the current conditions in the watershed. The model was calibrated for 

the year 2000, which represented both high and low flow periods. In 2000, flows were 

very low and near critical 7Q10 conditions, while in 2001 flows were higher.

TMDL Bayou Sara/Norton Creek – Mobile River Basin Organic Enrichment/DO 

(ADEM 1996). Summer (May –November) TMDL critical conditions and MOS were 

established as 7Q10 flows and 30°C. The winter (December –April) TMDL critical 

conditions and MOS were established as 7Q2 and 20 °C.

Total Maximum Daily Load Cooper River, Wando River, Charleston Harbor 

System, South Carolina (SCDHEC 2002). Critical conditions for this dissolved oxygen 

TMDL were determined in the model by setting water quality parameters to represent 

75/25 percentiles. The average spring and neap tidal conditions were evaluated with 

fresh water inflow set to approximate a 7Q10 recurrence, and algal processes were turned

off. The model was calibrated to a three-day period and validated on a two-day period in 

1993. The seasonal critical period was considered to be the low flow, high temperature 

conditions associated with summer and early fall. 

Total Maximum Daily Load Ashley River, South Carolina. (SCDEHC 2003). The 

recommended critical flow period includes setting uncontrolled freshwater inflows to 

7Q10 flows and selecting the seaward tidal boundary to represent a full lunar month 
Draft Appendix G – Chesapeake Bay TMDL for Nutrients and Sediment

G-4 September 24, 2010

including both spring and neap tides. These conditions approach worst-case conditions 

for the impact of point sources on river DO levels. The wasteloads determined for these 

critical conditions are considered to be protective of the river DO standard when river 

flow is equal to or greater than 7Q10 since higher flows would provide greater dilution. 

Higher river flows are expected during wet weather, so the wasteloads should be 

protective under these conditions.

Another common method for determining the critical period was the selection of a three-year 

time span based on precipitation, selected to include a wet year, a dry year and a normal year. 

Some examples of this approach include:

Total Maximum Daily Load Analysis for Indian River, Indian River Bay and 

Rehoboth Bay, Delaware (DNREC 1998). This is a nitrogen and phosphorus TMDL. 

The baseline period was established as 1988 through 1990. The hydrologic condition of 

the year 1988 was considered to represent a dry year, 1989 a wet year, and 1990 a normal 

year. No indication of the full data set from which the baseline period was established 

was given. 

Total Maximum Daily Loads of Nitrogen and Phosphorus for Baltimore Harbor in 

Anne Arundel, Baltimore, Carroll, and Howard Counties and Baltimore City, 

Maryland (MDE 2006). The baseline conditions scenario represents the observed 

conditions of the Harbor and its tributaries from 1995-1997. Simulating the system for 

three years accounts for various loading and hydrologic conditions, which represent 

possible critical conditions and seasonal variations of the system. For example, the 1995-

1997 period includes an average year (1995), a wet year (1996) and a dry year (1997).

Total Maximum Daily Load Organic Enrichment/Dissolved Oxygen Threemile 

Creek, Alabama (ADEM 2006). The hydrology of the LSPC model was calibrated for 

the period of record, October 1, 1996 through September 30, 2000. The time period of 

the model simulation was from 2000 to 2001. This time period was selected based on the 

availability and relevance of the observed data to the current conditions in the watershed. 

The model was calibrated for the year 2000, which represented both high and low flow 

periods. The model was simulated from May 2000 through April 2001 to account for 

both summer (May through November) and winter (December through April) conditions. 

In the natural conditions model, two critical periods were selected to establish seasonal 

TMDLs. A period during June 2000 was simulated under natural conditions which

resulted in a minimum DO concentration of 1.91 mg/L at a 5 ft depth. This June event 

defines critical conditions in Threemile Creek during the summer season. A period during 

April of 2001, the model simulated natural condition is 2.26 mg/L at a 5 ft depth and 

defines the winter critical period. A low flow period with high temperatures for both 

summer and winter seasons was utilized to represent the worst-case conditions.

Total Maximum Daily Loads of Nutrients/Biochemical Oxygen Demand for the 

Anacostia River Basin, Montgomery and Prince George’s Counties, Maryland and 

The District of Columbia. (MDE and District of Columbia Department of the 

Environment 2008). The critical condition and seasonality was accounted for in the 

TMDL analysis by the choice of simulation period, 1995-1997. This three-year time 
Draft Appendix G – Chesapeake Bay TMDL for Nutrients and Sediment

G-5 September 24, 2010

period represents a relatively dry year (1995), a wet year (1996), and an average year

(1997), based on precipitation data, and accounts for various hydrological conditions 

including the critical condition. 

Two TMDLs used the period of the worst hypoxia as the critical period. Dissolved oxygen 

exceedances for Long Island Sound s were dominated by point sources. Further details 

regarding the TMDLs include:

A Total Maximum Daily Load Analysis to Achieve Water Quality Standards for 

Dissolved Oxygen in Long Island Sound (NYSDEC and CTDEP 2000). Annual 

surveys from 1986-1998 and a review of historical data indicated that the 1988-1989 

modeling time frame was the most severe period of hypoxia on record. As a result, 

model simulations of reduced nitrogen inputs were used to predict water quality 

conditions that would result during the same physical conditions that exist during the 

1988-89 period. The use of 1988-89 worst case scenario was considered an implicit 

margin of safety. 

Total Maximum Daily Load for Nitrogen in the Peconic Estuary Program Study 

Area Including Waterbodies Currently Impaired Due to Low Dissolved Oxygen: 

the Lower Peconic River and Tidal Tributaries; Western Flanders Bay and Lower 

Sawmill Creek; and Meetinghouse Creek, Terrys Creek and Tributaries (Peconic 

Estuary Program 2007). The Environmental Fluid Dynamics Code (EFDC) model 

was calibrated using an eight-year period from October 1, 1988 to September 30, 1996

and validated using the six-year period from October 1, 196 through September 30, 

2002. Model calibration and verification included all seasons of the year, as well as 

extreme wet and dry years. Monitoring data indicated that the October 2000 to 

September 2002 time frame was the most severe period of hypoxia on record from

1988-2002. October 1, 2000 to September 30, 2002 was selected as the critical period 

for the TMDL model runs.

In some cases, the data set either does not contain a critical year or several years are included to

capture a range of temperature and flow concentrations. The TMDLs for The Little Assawoman 

Bay and Tributaries and Ponds of the Indian River, Indian River Bay, and Rehoboth Bay

(DNREC 2004) is an example of the former. There was no “worst” year for dissolved oxygen, 

nitrogen and phosphorus during the three-year period in question, so the average over the three 

summers was used as the critical (design) condition. The TMDL for Nutrients in the Lower 

Charles River Basin, Massachusetts (MassDEP and USEPA 2007) is an example of the latter. A 

continuous five-year simulation was run. The 1998-2002 period was selected because it 

represented some of the lowest summer flows throughout the 23 period of record. Low flows at 

or near the 7Q10 flow value were observed during three of the summers during the selected 

critical period. 

Two of the TMDLs reviewed had limited data sets, so the critical period was chosen based on the 

period with the most data available. Examples of this approach include:

Total Maximum Daily Loads of Nitrogen and Phosphorus for the Upper and Middle 

Chester River, Kent and Queen Anne’s Counties, Maryland (MDE 2006). The 
Draft Appendix G – Chesapeake Bay TMDL for Nutrients and Sediment

G-6 September 24, 2010

models were calibrated to the period of 1997-1999, which was the most recent period for 

which all of the needed data were available and consistent with the Chesapeake Bay 

Program modeling efforts of the Tributary Strategies. Only the output from 1997 was 

used to investigate different nutrient loading scenarios and calculate the annual average 

and growing season TMDLs for the Upper and Middle Chester Rivers because in 1999, 

the region experienced extreme weather conditions (prolonged drought followed by 

Hurricane Floyd) resulting in atypically high flows and loads. Based on the flow gauge, 

it was determined that the flow in 1997 was representative of the average annual flow and 

loads. The timeframe selected includes representative wet and dry periods, accounting 

for seasonality and critical conditions.

Total Maximum Daily Load for Dissolved Oxygen in Mill Creek, Northampton 

County, Virginia (VADEQ 2009). The observations show that the instantaneous DO 

levels fell below the water quality criterion of 4 mg/L minimum repeatedly throughout 

the period of 1997-2003. Because the nutrients data in the watershed were not available, 

an interactive approach of calibration of watershed and in-stream water quality model 

was conducted using all available in-stream monitoring data. The water quality model 

was calibrated in Mill Creek using the observation data. A six-year model simulation 

(1998-2003) was conducted. Seasonal variations involved changes in surface runoff, 

stream flow, and water quality condition as a result of hydrologic and climatologic 

patterns. These were accounted for by the use of this long-term simulation to estimate the 

current load and reduction targets.

Initial Analysis by Malcolm Pirnie

The consulting firm Malcolm Pirnie, representing the stakeholders from the Maryland 

Association of Municipal Wastewater Agencies, Inc (MAMWA) and the Virginia Association of 

Municipal Wastewater Agencies, Inc. (VAMWA) conducted an independent analysis of the 

inflows to the Chesapeake Bay to determine whether the initially selected critical period of 1996-

1998 may represent a hydrologic condition with a longer return period than 10 years (Malcolm 

Pirnie 2009). 

Malcolm Pirnie analyzed the flows from the Potomac River and the Susquehanna River, which 

together contribute most of the flow to the Chesapeake Bay, for the period 1967 through 2009. 

The average daily inflow from January through May was calculated for each year and for each 

three-year period within the 42-year period of record. January through May was selected as the 

period of interest because studies have indicated that the magnitude and extent of hypoxia in the 

Chesapeake Bay is largely controlled by freshwater and nutrient inputs during the preceding 

winter and spring months (freshet). 

Results indicated that 1996-1998 had the highest average January through May inflow over the 

entire period of record and would result in a return period of 40 years. The year 1996 had 

January through May inflows in the 93rd percentile and 1998 had flows in the 98th percentile. 

High flows in 1996 were attributed to rainfall on winter snowpack in January 1996, resulting in 

an event know as the “Big Melt.”
Draft Appendix G – Chesapeake Bay TMDL for Nutrients and Sediment

G-7 September 24, 2010

Based on these results, Malcolm Pirnie indicated that the critical condition would be too extreme 

if 1996-1998 is selected as the critical period. Malcolm Pirnie recommended using 1993-1995 or 

1994-1996 as the critical period because they represent return flows much closer to a 10-year 

return period. 

Replication of Malcolm Pirnie Results

To confirm the results of the Malcolm Pirnie analysis, Tetra Tech staff replicated the approach 

used in the Malcolm Pirnie flow analysis. The analysis was repeated using both the flow data 

presented in the Malcolm Pirnie technical memo (Malcolm Pirnie 2009) and the raw flow data

from the USGS. Although the replicated three-year averages based on the flows in the technical 

memo did not match exactly what was presented in the technical memo, the minor discrepancies 

did not affect the percentile calculations. Similarly, the three-year running averages using the 

raw USGS data resulted in minor discrepancies from the Malcolm Pirnie results. Despite the 

small differences, Tetra Tech’s replication yielded the same results as the Malcolm Pirnie 

technical memo (Malcolm Pirnie 2009). 

Analysis to Support Critical Period Selection

Additional analyses were performed to further explore the options for the selection of the critical 

period. 

Preliminary analysis included an exploration of the results of including the nine major rivers in the flow 

analysis and expanding the combinations of different monthly flow durations beyond January to May to 

include other monthly duration combinations from September through July. Data was analyzed for 1978 

through 2009 because the Patuxent flow gage did not begin until 1977. Refer to Table G-1 for the gages used 

in the analysis and the time period for which data was available. Running three-year average flows were 

calculated for 25 different month combinations for the entire period of evaluation. The probability of each 

three-year flow average was determined using the Weibull Plotting Position. The return period is the inverse 

of the probability. This method differed from the approach in the Malcolm Pirnie analysis (Malcolm Pirnie 

2009), which used percentile ranks. A regression was also performed on the three-year flow averages to 

determine if there was a correlation with the dissolved oxygen percent exceedances. The percent dissolved 

oxygen exceedances were provided by the EPA Chesapeake Bay Program Office (CBPO) and represent 

volume exceedances. The analysis was run with and without the use of tributary multipliers, which were 

developed by the CBPO because flows from different tributaries do not impact conditions in the Bay equally. 

These factors are the estuarine delivery factors presented in the section 6.9.2.1. The CBPO multipliers were 

translated to a 0.0 to 1.0 scale and are included in 

Table G-2. Without the multipliers the Susquehanna and Potomac Rivers contribute approximately 80 

percent of the flow to the Bay. With the multipliers, the two rivers contribute approximately 95 percent of the 

effective load. (

Figure G-1 and Figure G-2). 

Table G-1. Flow Gages and Time Period of Available Data.

Gage ID Description Start End 

1668000 Rappahannock River near Fredericksburg, VA 9/19/1907 8/25/2009 

1646502 Potomac River (Adjusted) near Washington, DC 3/1/1930 7/31/2009 
Draft Appendix G – Chesapeake Bay TMDL for Nutrients and Sediment

G-8 September 24, 2010

2037500 James River near Richmond, VA 10/1/1934 8/25/2009 

1674500 Mattaponi River near Beulahville, VA 9/19/1941 8/25/2009 

1673000 Pamunkey River near Hanover, VA 10/1/1941 8/25/2009 

1491000 Choptank River near Greensboro, MD 1/1/1948 8/25/2009 

1578310 Susquehanna River at Conowingo, MD 10/1/1967 8/25/2009 

2041650 Appomattox River at Matoaca, VA 10/1/1969 8/25/2009 

1594440 Patuxtent River near Bowie, MD 6/27/1977 8/25/2009 

Table G-2. Chesapeake Bay tributaries flow multiplier ratios.

Major River Basin Multiplier Adjusted Ratio

Appomattox 0.533111028 0.017

Choptank 6.929861533 0.217

James 0.533111028 0.017

Mattaponi 0.798423188 0.025

Pamunkey 0.798423188 0.025

Patuxent 3.093385849 0.097

Potomac 6.188243619 0.193

Rappahannock 2.809613056 0.088

Susquehanna 10.3187158 0.322

1.000

Source: EPA Chesapeake Bay Program Office.
Draft Appendix G – Chesapeake Bay TMDL for Nutrients and Sediment

G-9 September 24, 2010 1978-2009

MATTAPONI, 0.8%

PAMUNKEY, 1.5%

PATUXENT, 0.6%

SUSQUEHANNA, 61.3%

RAPPAHANNOCK, 2.7%

JAMES, 11.2% CHOPTANK, 0.2% APPOMATTOX, 2.0%

POTOMAC_502, 19.7%

Figure G-1. Tributary flow contributions without multiplier ratios.

1978-2009

After Multipliers

POTOMAC_502, 15.8%

APPOMATTOX, 0.1%

CHOPTANK, 0.2%

JAMES, 0.8%

RAPPAHANNOCK, 1.0%

SUSQUEHANNA, 81.7%

MATTAPONI, 0.1%

PAMUNKEY, 0.2%

PATUXENT, 0.2%

Figure G-2. Tributary flow contributions with the multiplier ratios.
Draft Appendix G – Chesapeake Bay TMDL for Nutrients and Sediment

G-10 September 24, 2010

Results of the analysis, as shown in Table G-3 and Table G-4, indicated that the monthly span 

should be extended beyond the January through May period suggested in the Malcolm Pirnie 

analysis (Malcolm Pirnie 2009) because the three-year flow averages with the highest correlation 

to dissolved oxygen exceedances generally included longer monthly spans. The three-year 

average flow with the highest correlation to DO exceedances was September through June. 

Findings also suggested that 1996-1998 had closer to a 15-year return period for months when 

flow was more closely correlated with dissolve oxygen exccedances. The other possible critical 

periods 1992-1994 and 1993-1995 had generally lower than 10-year return periods and return 

periods greater than 10 years when flow was not strongly correlated with DO exceedances. 

Return periods greater than six years are highlighted in Tables G-3 and G-4 and only three-year 

average flows with at least one monthly interval with a six-year or greater return period are 

shown. There were no three-year average flows with return periods greater than six years for 

any of the years between 1978 and 1991.

Table G-3. Return periods and R2correlation between various monthly durations and dissolved 

oxygen percent exceedances without the Tributary Multiplier Ratio.

% DO Exceedences ---> 25.87% 25.92% 24.26% 27.84% 26.05% 31.11% 27.24%

Interval R2 1992-1994 1993-1995 1994-1996 1996-1998 1997-1999 2003-2005 2004-2006

SEP-JUNE 0.54 4.43 6.20 3.44 15.50 2.58 31.00 7.75

NOV-JUNE 0.53 6.20 7.75 5.17 31.00 2.07 15.50 4.43

SEP-JULY 0.53 4.43 5.17 3.44 15.50 2.58 31.00 10.33

NOV-JULY 0.52 6.20 7.75 4.43 15.50 2.07 31.00 5.17

DEC-JUNE 0.52 7.75 6.20 4.43 31.00 2.38 15.50 3.88

SEP-MAY 0.51 4.43 6.20 3.88 15.50 3.10 31.00 7.75

DEC-JULY 0.51 6.20 7.75 4.43 31.00 2.21 15.50 3.88

OCT-JUNE 0.50 5.17 6.20 4.43 15.50 2.38 31.00 7.75

OCT-JULY 0.49 5.17 6.20 4.43 15.50 2.21 31.00 7.75

NOV-MAY 0.48 6.20 7.75 5.17 31.00 3.10 15.50 4.43

SEP-APR 0.48 4.43 5.17 3.44 15.50 3.10 31.00 10.33

OCT-MAY 0.46 5.17 7.75 4.43 31.00 2.82 10.33 6.20

DEC-MAY 0.46 10.33 7.75 5.17 31.00 2.82 6.20 3.88

JAN-JUNE 0.44 10.33 6.20 4.43 31.00 2.58 5.17 2.21

JAN-JULY 0.44 6.20 5.17 4.43 31.00 2.21 7.75 2.82

NOV-APR 0.44 7.75 10.33 4.43 31.00 3.10 15.50 5.17

OCT-APR 0.42 5.17 7.75 3.44 31.00 3.10 15.50 6.20

SEP-MAR 0.42 2.82 3.44 3.88 15.50 4.43 31.00 10.33

DEC-APR 0.40 10.33 15.50 5.17 31.00 3.10 6.20 4.43

NOV-MAR 0.39 3.10 3.44 6.20 31.00 4.43 15.50 7.75

JAN-MAY 0.37 10.33 7.75 6.20 31.00 3.10 4.43 2.21

OCT-MAR 0.36 2.82 3.44 4.43 31.00 3.88 10.33 7.75

DEC-MAR 0.36 3.44 5.17 7.75 31.00 4.43 10.33 6.20

JAN-APR 0.32 31.00 15.50 6.20 10.33 3.44 3.88 2.38

JAN-MAR 0.26 5.17 6.20 10.33 31.00 7.75 3.88 2.58

Return Period
Draft Appendix G – Chesapeake Bay TMDL for Nutrients and Sediment

G-11 September 24, 2010

Table G-4. Return periods and R2correlation between various monthly durations and dissolved 

oxygen percent exceedances with the Tributary Multiplier Ratio.

% DO Exceedences ---> 25.87% 25.92% 24.26% 27.84% 26.05% 31.11% 27.24%

Interval R2 1992-1994 1993-1995 1994-1996 1996-1998 1997-1999 2003-2005 2004-2006

SEP-JUNE 0.53 4.43 5.17 3.44 7.75 2.21 31.00 15.50

NOV-JUNE 0.53 5.17 6.20 4.43 15.50 1.94 31.00 7.75

DEC-JUNE 0.52 6.20 7.75 3.88 15.50 1.94 31.00 4.43

SEP-JULY 0.52 3.88 5.17 3.44 10.33 2.07 31.00 15.50

NOV-JULY 0.52 5.17 6.20 4.43 15.50 1.94 31.00 10.33

DEC-JULY 0.51 5.17 6.20 3.88 15.50 1.94 31.00 7.75

OCT-JUNE 0.49 5.17 6.20 3.88 15.50 2.07 31.00 7.75

SEP-MAY 0.49 4.43 5.17 3.88 7.75 2.58 31.00 15.50

OCT-JULY 0.48 5.17 6.20 3.88 15.50 1.94 31.00 10.33

NOV-MAY 0.46 6.20 7.75 4.43 31.00 2.38 15.50 5.17

SEP-APR 0.46 4.43 5.17 3.44 6.20 2.82 31.00 15.50

JAN-JULY 0.46 10.33 5.17 4.43 31.00 1.55 15.50 3.88

JAN-JUNE 0.46 10.33 6.20 4.43 31.00 1.82 5.17 2.82

DEC-MAY 0.45 7.75 10.33 5.17 31.00 2.21 6.20 4.43

OCT-MAY 0.44 5.17 6.20 3.88 15.50 2.21 10.33 7.75

NOV-APR 0.42 7.75 10.33 3.88 15.50 2.58 31.00 6.20

SEP-MAR 0.41 2.07 3.10 3.88 10.33 4.43 15.50 31.00

OCT-APR 0.41 5.17 6.20 3.44 10.33 2.58 31.00 7.75

DEC-APR 0.40 15.50 31.00 4.43 10.33 2.58 7.75 5.17

NOV-MAR 0.38 2.58 3.10 5.17 31.00 3.44 15.50 10.33

JAN-MAY 0.37 15.50 7.75 6.20 31.00 2.38 5.17 2.82

DEC-MAR 0.37 2.58 3.44 6.20 31.00 3.88 15.50 10.33

OCT-MAR 0.35 2.38 3.10 4.43 31.00 3.44 10.33 15.50

JAN-APR 0.32 31.00 15.50 6.20 10.33 2.58 5.17 3.44

JAN-MAR 0.28 2.58 3.88 10.33 31.00 7.75 6.20 2.82

Return Period

Analysis of Critical Period Using the Log Pearson III Method

After determining the return period using the Weibull Plotting Position method, a second method, the Log 

Pearson III Method (U. S. Interagency Advisory Committee on Water Data 1982; Ponce 1989), was used 

to determine whether the return period changed significantly depending on the method of calculation. 

The Log Pearson III method provides a smooth fit through the plotting position data and in essence 

smoothens out the predicted values. This analysis was conducted over the same 1978 through 2009 time 

period and focused on monthly spans with the highest correlation between flow and DO exceedances. 

Results in Table G-5 and Table G-6 show that there are some changes in the return periods, but the 

conclusion in terms of candidate years remains the same. This method of determining the return period 

was used in subsequent analyses. 
Draft Appendix G – Chesapeake Bay TMDL for Nutrients and Sediment

G-12 September 24, 2010

Table G-5. Log Pearson III method for determining return period, without Tributary Multiplier Ratio.

Without Multiplier

% DO Exceedences 25.87% 25.92% 24.26% 27.84% 31.11% 27.24%

Year 1992-1994 1993-1995 1994-1996 1996-1998 2003-2005 2004-2006

Sep-June 4.38 4.90 3.77 17.99 34.80 12.37

Nov-June 7.45 7.90 5.46 20.71 19.09 5.36

Sep-July 4.16 4.79 4.05 16.77 36.03 14.15

Nov-July 6.79 7.53 6.02 18.95 20.33 6.59

Dec-June 9.19 9.11 6.68 19.70 15.89 4.24

Sep-May 4.90 5.74 3.80 17.77 23.83 11.69

Dec-July 8.39 8.66 7.26 18.14 17.24 4.97

Oct-June 5.44 6.15 4.60 19.99 21.57 7.16

Flow (Sep-June) (cfs) 81,791 83,254 80,099 95,684 101,516 92,106

Flow (Nov-June) (cfs) 97,725 98,368 94,810 108,161 107,300 94,664

Flow (Sep-July) (cfs) 76,755 78,432 76,487 89,677 96,200 88,110

Flow (Nov-July) (cfs) 89,756 90,753 88,724 99,399 100,142 89,485

Flow (Dec-June) (cfs) 104,233 104,117 100,461 111,988 109,418 95,653

Flow (Sep-May) (cfs) 86,706 88,203 83,278 100,501 103,783 96,146

Flow (Dec-July) (cfs) 94,451 94,829 92,906 101,658 101,107 89,709

Flow (Oct-June) (cfs) 88,780 89,746 87,057 101,106 101,688 91,140

Table G-6. Log Pearson III method for determining return period, with Tributary Multiplier Ratio.

With Multiplier

% DO Exceedences 25.87% 25.92% 24.26% 27.84% 31.11% 27.24%

Year 1992-1994 1993-1995 1994-1996 1996-1998 2003-2005 2004-2006

Sep-June 4.39 5.17 3.87 13.21 35.52 18.76

Nov-June 7.47 8.19 5.70 16.84 19.21 8.52

Sep-July 4.19 4.83 4.04 12.21 36.18 21.53

Nov-July 6.85 7.48 5.98 16.06 21.37 10.34

Dec-June 9.17 9.27 6.76 16.02 17.64 6.88

Sep-May 4.92 6.32 4.08 13.12 24.42 17.15

Dec-July 8.38 8.39 7.08 14.58 18.76 8.73

Oct-June 5.40 6.41 4.67 16.09 22.11 10.74

Flow (Sep-June) (cfs) 19,682 20,141 19,338 22,251 24,445 23,100

Flow (Nov-June) (cfs) 23,429 23,668 22,837 25,294 25,648 23,779

Flow (Sep-July) (cfs) 18,494 18,892 18,400 20,891 23,136 22,147

Flow (Nov-July) (cfs) 21,550 21,739 21,292 23,285 23,910 22,535

Flow (Dec-June) (cfs) 24,860 24,893 24,069 26,006 26,242 24,110

Flow (Sep-May) (cfs) 20,897 21,462 20,265 23,415 25,103 24,122

Flow (Dec-July) (cfs) 22,568 22,569 22,178 23,659 24,214 22,671

Flow (Oct-June) (cfs) 21,337 21,662 20,998 23,689 24,436 22,921

Analysis of Critical Period Using Expanded Flow Data

Given some concern that the 30-year period from 1978 through 2009 was of insufficient length to fully 

capture the return period over the full time period of flow data and was artificially lowering the most 

extreme return period to 30 years, an extended analysis was performed for the years 1930 through 2009 

but only included the Potomac and Susquehanna rivers. The Potomac and Susquehanna rivers account for 

almost 80 percent of the total flow to the Chesapeake Bay, and if the CBPO allocation multipliers are 

used these two rivers account for almost 95 percent of the total inflow to the Chesapeake Bay. Hence, 

these two flow gages were considered sufficient for analysis purposes. These two USGS flow gages were 

previously described in Table G-1. 
Draft Appendix G – Chesapeake Bay TMDL for Nutrients and Sediment

G-13 September 24, 2010

The Susquehanna River at Conowingo gage flow data runs from October 1, 1967 to the present. 

The period prior to October 1, 1967 was patched using data from the Susquehanna River at 

Harrisburg gage (01570500 – October 1, 1890 to August 25, 2009) using a simple drainage area 

ratio method. The daily fresh water inflow from the Potomac River and the Susquehanna River 

were weighed using the adjusted tributary multipliers provided by the CBPO (Table G-7).

Table G-7. Adjusted tributary flow multiplier ratios.

Gage Multiplier Adjusted Ratio

Potomac 6.188 0.375

Susquehanna 10.317 0.625

Source: EPA Chesapeake Bay Program Office.

The analysis using the extended time period followed the same procedure as previous analyses 

except that the data was extended back to 1930, only the weighted flow data based on multipliers 

was used, and the Log Pearson III method was used to determine the return period. Table G-8

lists the return periods for each of the monthly intervals for the extended time period, with return 

periods greater than six years highlighted. 

Table G-8. Extended time period (1930-2009) return periods.

% DO Exceedences 24.97% 25.87% 25.92% 24.26% 22.58% 27.84% 31.11% 27.24%

Year 1991-1993 1992-1994 1993-1995 1994-1996 1995-1997 1996-1998 2003-2005 2004-2006

jan-july 2.69 11.80 8.95 8.72 1.77 16.28 11.76 4.37

jan-june 3.05 13.72 9.84 8.14 1.69 17.59 9.71 3.03

jan-may 4.61 24.98 19.13 10.56 1.69 25.43 7.20 2.73

jan-apr 7.48 39.45 34.34 10.82 1.81 16.67 7.48 3.59

jan-mar 2.18 3.24 4.32 13.91 4.28 46.60 5.51 4.33

dec-july 3.03 9.20 9.15 7.92 2.69 15.66 20.18 9.88

dec-june 3.35 9.90 9.98 7.52 2.62 17.02 19.14 7.95

dec-may 4.76 16.77 17.73 9.20 2.76 23.09 16.70 8.14

dec-apr 6.96 20.14 23.89 9.10 3.01 16.01 16.48 9.99

dec-mar 2.68 3.49 5.42 9.87 7.27 31.16 13.94 13.66

nov-july 1.66 2.08 3.29 2.63 3.11 2.75 1.35 1.31

nov-june 3.39 8.92 9.67 7.10 3.18 20.60 25.44 10.69

nov-may 4.68 13.11 15.60 8.48 3.43 28.01 21.32 11.48

nov-apr 6.51 16.24 19.83 8.46 3.78 19.26 21.02 15.07

nov-mar 2.84 3.43 5.51 8.90 8.28 34.04 17.98 17.83

oct-july 3.64 6.50 7.38 6.27 3.71 18.35 32.07 18.23

oct-june 4.12 6.98 8.03 5.91 3.72 19.90 31.72 15.37

oct-may 5.69 9.02 10.95 7.06 4.09 25.80 26.88 16.45

oct-apr 7.66 10.82 14.96 7.08 4.40 18.91 26.38 19.62

oct-mar 3.42 2.92 4.50 7.25 8.82 29.23 20.77 22.25

sep-july 3.39 5.40 6.73 5.06 4.18 17.56 69.44 38.08

sep-june 3.86 5.81 7.27 4.87 4.26 18.29 62.21 30.68

sep-may 4.93 7.51 9.31 5.64 4.62 21.90 56.34 34.77

sep-apr 6.60 8.70 11.93 5.68 4.90 17.28 52.38 40.22

sep-mar 3.25 2.74 4.31 5.78 9.16 23.34 40.15 43.20

The monthly intervals with high correlations with DO exceedances are September – June, 

November – June, December – June, September – July and December – July. Table G-9

highlights the return periods for the monthly intervals with high correlations with DO 

exceedances. 
Draft Appendix G – Chesapeake Bay TMDL for Nutrients and Sediment

G-14 September 24, 2010

Table G-9. Return periods for monthly intervals highly correlated to Chesapeake Bay dissolved 

oxygen criteria exceedances.

Interval 1992-1994 1993-1995 1994-1996 1996-1998

September – June 5.81 7.27 4.87 18.29

November – June 8.92 9.67 7.10 20.60

December – June 9.90 9.98 7.52 17.02

September - July 5.40 6.73 5.06 17.56

December - July 9.20 9.15 7.92 15.66

Analysis of Critical Period using De-Trended Flow Data

As was previously noted, initial analysis of the three-year average flows from 1978 through 2009 

did not reveal any three year periods prior to 1992 with return periods greater than six years for 

the monthly intervals included in the analysis. This indicates a potential increasing trend in flow 

volume over the last several decades. De-trending removes any flow trends over time and allows 

for an equal comparison of current and historic flows. It can remove the effects of urbanization 

and other impacts, which are apparent in the flow data. 

The first step in de-trending was to determine if there is a significant trend in the flow data. The 

slope of the trend line is 0.1878. The Kendall Tau ranking correlation coefficient was used to 

determine if this is a statistically significant trend. The Tau value can range between -1 and 1 

with a positive number indicating an increasing trend and a negative number indicating a 

decreasing trend. The flow data from 1930 through 2009 had a positive Tau value. A p-value 

<0.05 indicates a statistically significant trend. The time-series flow data had a p-value of 

0.0042, which is statistically significant. Figure G-3 shows the trend line in the raw data.

After establishing there is a statistically significant increasing trend in the flow data, a de-trended 

time-series was developed. Two different methods were used to fit a trend line through the timeseries data – Linear Least Squares Regression, and the Locally Weighted Scatter Plot Smoothner 

(LOWESS) (Helsel and Hirsch 2002 and NIST/SEMATECH). 

The linear regression trend line was estimated by fitting the time-series data using a trend line of 

the form y=mx+c (where m is the slope, c is the intercept, y being the dependent variable i.e. 

flow and x the independent variable time). The LOWESS fit is determined by specifying a 

smoothening parameter which defines the subset of data which will be used for the local fit. The 

LOESS technique performs a weighted least square regression fit (on a subset of points) in a 

moving range around the X value (time), where the values in the moving range are weighted 

according to their distance from this X value. For this analysis a smoothning parameter of 0.33 

was found to fit the data trend reasonably well. Details of the LOWESS computation can be 

found at: http://www.itl.nist.gov/div898/handbook/pmd/section1/dep/dep144.htm.

The residuals were then calculated for each method (i.e. the difference between the observed and 

predicted values along the trend line). Finally the residuals were added to the last point in the 

time series (the maximum value) to generate a de-trended time series. To confirm that no trend 

exists in the resulting de-trended time series using the linear regression approach, the linear slope 

was calculated. The slope was zero, indicating that there was no remain trend. For the de-
Draft Appendix G – Chesapeake Bay TMDL for Nutrients and Sediment

G-15 September 24, 2010

trended time-series using the LOWESS regression the presence of no trend in the time-series was 

confirmed using a p-value. The p-value of the de-trended data was 1.2376, indicating a 

statistically insignificant trend (p-value < 0.05 is significant). Figure G-4 plots the de-trended 

data. 

Figure G-3. Raw flow data with trend line.

Figure G-4. De-trended data with slope of zero
Draft Appendix G – Chesapeake Bay TMDL for Nutrients and Sediment

G-16 September 24, 2010

Linear Regression to Determine Return Period

Using the linear regression de-trended data yielded revised return periods, which can be found in 

Table G-10. Table G-11 highlights return periods for the monthly spans with the highest 

correlation to dissolved oxygen exceedances. 

Table G-10. De-trending analysis results using linear regression.

% DO Exceedences 25.87% 25.92% 24.26% 22.58% 27.84% 31.11% 27.24%

Year 1992-1994 1993-1995 1994-1996 1995-1997 1996-1998 2003-2005 2004-2006

jan-july 7.53 5.51 5.14 1.41 9.02 6.05 2.49

jan-june 8.57 6.62 4.89 1.39 9.82 5.28 1.97

jan-may 16.31 11.91 6.84 1.41 15.37 3.88 1.85

jan-apr 26.99 22.35 7.73 1.54 10.27 4.50 2.46

jan-mar 2.67 3.36 9.85 3.28 34.34 3.92 3.10

dec-july 6.52 6.34 4.95 1.95 9.54 11.75 5.74

dec-june 7.38 7.36 4.83 1.95 10.73 11.13 4.48

dec-may 11.05 11.80 6.33 2.06 15.37 9.18 4.57

dec-apr 16.93 19.29 6.92 2.28 11.43 10.39 6.93

dec-mar 2.83 4.30 8.35 5.44 26.43 9.67 9.45

nov-july 2.80 4.80 3.61 4.36 3.69 1.46 1.41

nov-june 6.35 7.03 4.60 2.29 14.35 15.47 6.38

nov-may 9.00 10.18 5.63 2.44 19.11 13.24 6.80

nov-apr 12.56 16.41 6.16 2.77 15.06 14.98 9.32

nov-mar 2.75 4.30 7.17 6.40 29.15 13.42 13.06

oct-july 4.31 4.71 4.05 2.48 12.57 19.18 9.92

oct-june 4.64 5.26 3.96 2.58 13.94 18.36 8.54

oct-may 6.42 7.83 4.53 2.79 18.18 16.63 9.13

oct-apr 8.37 10.70 4.77 3.12 14.50 18.16 13.31

oct-mar 2.29 3.42 5.25 6.88 23.92 15.97 16.78

sep-july 3.75 4.39 3.45 2.81 11.30 40.03 21.57

sep-june 4.00 4.73 3.31 2.87 13.01 42.41 18.94

sep-may 4.91 6.67 3.79 3.13 16.03 37.44 20.99

sep-apr 6.53 8.84 4.01 3.48 12.77 39.63 29.60

sep-mar 2.14 3.23 4.29 7.21 19.30 32.86 34.85

Table G-11: Return periods for monthly intervals highly correlated to Chesapeake Bay dissolved 

oxygen criteria exceedances using linear regression de-trended flow data.

Interval 1992-1994 1993-1995 1994-1996 1996-1998

September – June 4.00 4.73 3.31 13.01

November – June 6.35 7.03 4.60 14.35

December – June 7.38 7.36 4.83 10.73

September - July 3.75 4.39 3.45 11.30

December - July 6.52 6.34 4.95 9.54

LOWESS Polynomial Regression

Using LOWESS regression to de-trend the data, the three-year return periods were recalculated 

(Tables G-12 and G-13). 
Draft Appendix G – Chesapeake Bay TMDL for Nutrients and Sediment

G-17 September 24, 2010

Table G-12. De-trending analysis results using LOWESS polynomial regression.

% DO Exceedences 24.97% 25.87% 25.92% 24.26% 22.58% 27.84% 31.11% 27.24%

Year 1991-1993 1992-1994 1993-1995 1994-1996 1995-1997 1996-1998 2003-2005 2004-2006

jan-july 2.25 11.24 8.10 7.33 1.42 11.81 7.30 2.60

jan-june 2.57 13.21 9.07 6.67 1.40 13.47 6.26 1.98

jan-may 3.88 23.61 17.29 8.59 1.44 18.30 4.16 1.88

jan-apr 6.54 38.98 32.42 9.11 1.58 12.20 4.73 2.53

jan-mar 1.98 3.00 3.95 13.24 3.61 44.48 4.14 3.21

dec-july 2.61 9.21 8.92 7.01 2.06 12.92 15.91 7.02

dec-june 2.99 9.92 9.82 6.55 2.05 14.52 14.78 4.95

dec-may 4.23 17.41 18.11 8.19 2.15 19.58 11.04 4.92

dec-apr 6.39 21.35 25.19 8.30 2.44 13.25 12.00 7.63

dec-mar 2.39 3.18 4.99 9.93 6.51 35.53 11.54 11.12

nov-july 1.73 2.15 3.58 2.65 3.13 2.67 1.30 1.31

nov-june 3.02 8.93 9.61 6.16 2.47 18.92 19.92 7.68

nov-may 4.13 14.14 16.91 7.59 2.62 28.85 17.34 7.96

nov-apr 5.91 17.53 22.63 7.72 3.00 17.97 17.60 10.73

nov-mar 2.47 3.08 4.99 8.85 7.67 44.25 16.87 16.58

oct-july 3.16 6.30 7.20 5.28 2.81 18.23 31.63 14.98

oct-june 3.63 6.83 7.95 4.91 2.85 20.09 30.32 11.10

oct-may 4.95 9.06 11.49 5.97 3.06 28.12 23.30 11.96

oct-apr 7.36 11.36 16.16 6.17 3.45 17.97 22.69 16.49

oct-mar 3.10 2.57 4.14 6.83 8.28 33.96 19.30 20.54

sep-july 3.00 4.97 6.38 4.44 3.18 16.66 81.73 36.71

sep-june 3.32 5.35 7.02 4.21 3.24 18.26 82.60 29.70

sep-may 4.46 7.18 9.27 4.63 3.51 22.56 73.13 34.38

sep-apr 6.09 8.59 12.46 4.76 4.01 16.11 59.30 40.07

sep-mar 2.92 2.37 3.87 5.01 8.65 25.51 44.82 48.49

Table G-13: Return periods for monthly intervals highly correlated to Chesapeake Bay dissolved 

oxygen criteria exceedances using LOWESS polynomial regression de-trended flow data.

Interval 1992-1994 1993-1995 1994-1996 1996-1998

September – June 5.35 7.02 4.21 18.26

November – June 8.93 9.61 6.16 18.92

December – June 9.92 9.82 6.55 14.52

September - July 4.97 6.38 4.44 16.66

December - July 9.21 8.92 7.01 12.92

Summary of analyses

There is no strict guidance on determining the critical period and others have determined the 

critical period for TMDLs based on data availability, capturing the worst conditions in the period 

of record, capturing a range of flows, or 7Q10 flow. The availability of many decades of flow 

and water quality monitoring data in the Chesapeake Bay watershed allowed the opportunity to 

select a critical period from a group of candidate periods so there is some freedom to follow a 

very rational approach to the selection of the period. Of the above criteria that others have used 

to set critical periods, the idea of a 10-year return period is common and amenable to analysis.

The analyses presented here take into account two methods of calculating probability, two 

methods of giving weight to more effective basins, two time periods to calculate long term 

probability, and two de-trending methods. All methods are more or less relevant and are 

considered as a group to determine the critical period most indicative of a 10-year return period. 

Of the candidate periods, 1996-1998 and 1993-1995 are closest to the 10-year return period. 

Table G-14 below summarizes the results from these two candidate periods.
Draft Appendix G – Chesapeake Bay TMDL for Nutrients and Sediment

G-18 September 24, 2010

Table G-14. Summary of results for 1993-1995 and 1996-1998 periods.

All Tributaries (1978 - 2009) Potomac + Susquehanna (1930 - 2009)

Without 

Multiplier

With 

Multiplier

With 

Multiplier

With 

Multiplier

With 

Multiplier

No DetrendingNo DetrendingNo Detrending

De-trended 

(Linear 

regression)

De-trended 

(LOWESS)

Year 1993-1995

Median (High r

2

) 7.53 7.48 7.27 6.34 8.92

Mean (High r

2

) 6.84 6.99 7.39 5.97 8.35

Median (All monthly spans) 9.31 6.62 9.07

Mean (All monthly spans) 11.28 8.05 11.26

Overall Range 1993 - 1995 5.97-11.28

Year 1996-1998

Median (High r

2

) 18.95 16.02 17.56 11.3 16.66

Mean (High r

2

) 18.82 14.87 15.24 11.78 16.26

Median (All monthly spans) 19.26 14.35 18.26

Mean (All monthly spans) 21.63 15.57 21.05

Overall Range 1996 - 1998 11.30-21.63

Using the above table to compare 1993-1995 and 1996-1998, it is clear that in all methods of 

determining the return period, the 1996-1998 period has a return period of greater than 10 years. 

The period 1993-1995 is generally evaluated to be slightly below a 10-year return period, but the 

overall range incorporates the 10-year period. 1993-1995 was selected the Water Quality Goal 

Implementation Team as the most appropriate critical period because it was the most consistent 

with existing Chesapeake Bay watershed jurisdictions’ practices.
Draft Appendix G – Chesapeake Bay TMDL for Nutrients and Sediment

G-19 September 24, 2010

References

Alabama Department of Environmental Management (ADEM). 1996. TMDL Bayou 

Sara/Norton Creek – Mobile River Basin Organic Enrichment/DO.

ADEM. 2005. Organic Enrichment/Dissolved Oxygen TMDL Rabbit Creek and Dog River. 

ADEM. 2006. Total Maximum Daily Load Organic Enrichment/Dissolved Oxygen Threemile 

Creek, Alabama.

Delaware Department of Natural Resources and Environmental Control (DNREC). 1998. Total 

Maximum Daily Load Analysis for Indian River, Indian River Bay and Rehoboth Bay, 

Delaware. Dover, DE.

DNREC. 1998. Total Maximum Daily Load Analysis for Nanticoke River and Broad Creek, 

Delaware. Dover, DE.

DNREC. 2004. TMDLs for The Little Assawoman Bay and Tributaries and Ponds of the Indian 

River, Indian River Bay, and Rehoboth Bay. Prepared by ENTRIX, Inc. and J.E. Edinger 

Associates, Inc. New Castle, DE and Wayne, PA.

Helsel, D.R., and Hirsch, R.M.. 2002. Techniques of Water-Resources Investigations of the 

United States Geological Survey. Chapter A3: Statistical methods in water resources. Book 4, 

Hydrologic Analysis and Interpretation.

Malcolm Pirnie, Inc.. 2009. Technical Memorandum: Analysis of January-May Inflows to the 

Chesapeake Bay During the 1996-98 Period. 

MDE (Maryland Department of the Environment). 2006. Total Maximum Daily Loads of 

Nitrogen and Phosphorus for Baltimore Harbor in Anne Arundel, Baltimore, Carroll, and 

Howard Counties and Baltimore City, Maryland. Baltimore, MD.

MDE (Maryland Department of the Environment). 2006. Total Maximum Daily Loads of 

Nitrogen and Phosphorus for the Upper and Middle Chester River, Kent and Queen Anne’s 

Counties, Maryland. Baltimore, MD.

Massachusetts Department of Environmental Protection and US Environmental Protection 

Agency, New England Region (MassDEP and USEPA) 2007. TMDL for Nutrients in the Lower 

Charles River Basin, Massachusetts. Worcester and Boston, MA.

MDE and District of Columbia Department of the Environment. 2008. Total Maximum Daily 

Loads of Nutrients/Biochemical Oxygen Demand for the Anacostia River Basin, Montgomery 

and Prince George’s Counties, Maryland and The District of Columbia. Baltimore, MD.

National Institute of Standards and Technology (NIST) and SEMATECH. e-Handbook of 

Statistical Methods, http://www.itl.nist.gov/div898/handbook/, U.S. Commerce Department's 

Technology Administration. Last updated: 7/18/2006.

New York State Department of Environmental Conservation and Connecticut Department of 

Environmental Protection. 2000. A Total Maximum Daily Load Analysis to Achieve Water 

Quality Standards for Dissolved Oxygen in Long Island Sound. Albany, NY and Hartford, CT.
Draft Appendix G – Chesapeake Bay TMDL for Nutrients and Sediment

G-20 September 24, 2010

Peconic Estuary Program. 2007. Total Maximum Daily Load for Nitrogen in the Peconic Estuary 

Program Study Area Including Waterbodies Currently Impaired Due to Low Dissolved Oxygen: 

the Lower Peconic River and Tidal Tributaries; Western Flanders Bay and Lower Sawmill 

Creek; and Meetinghouse Creek, Terrys Creek and Tributaries. Yaphank, NY.

Ponce, V. M., 1989. Engineering Hydrology: Principles and Practices. ISBN 0-13-277831. 

Prentice-Hall, Inc.

SCDHEC (South Carolina Department of Health and Environmental Control). 2002. Total 

Maximum Daily Load Cooper River, Wando River, Charleston Harbor System, South Carolina. 

SCDHEC (South Carolina Department of Health and Environmental Control). 2003. Total 

Maximum Daily Load Ashley River, South Carolina.

VADEQ (Virginia Department of Environmental Quality). 2009. Total Maximum Daily Load for 

Dissolved Oxygen in Mill Creek, Northampton County, Virginia. Prepared by Virginia Institute 

of Marine Science. Gloucester Point, VA.

USEPA (U.S. Environmental Protection Agency). 2003a. Ambient Water Quality Criteria for 

Dissolved Oxygen, Water Clarity and Chlorophyll a for the Chesapeake Bay and Its Tidal 

Tributaries. EPA 903-R-03-002. U.S. Environmental Protection Agency, Region 3, Chesapeake 

Bay Program Office, Annapolis, MD.

U. S. Interagency Advisory Committee on Water Data, 1982, Guidelines for determining floodflow frequency: Bulletin 17B of the Hydrology Subcommittee, Office of Water Data 

Coordination, U.S. Geological Survey, Reston, Va., 183 p., 

http://water.usgs.gov/osw/bulletin17b/bulletin_17B.html