Why Is A Body of Water 

Impaired?

To help assessors identify possible causes of biological 

impairment of our waters, the U.S. EPA developed the 

CADDIS methodology and this web site. It is critical 

that causes be identified so that management efforts 

such as Total Maximum Daily Load (TMDL) calculations 

can be implemented and waters can be restored.

Office of Research and Development

U.S. Environmental Protection Agency

1200 Pennsylvania Ave NW

Washington, DC 20460

e-mail: caddis@epa.gov

www.epa.gov/caddis

Using EPA’s

Causal Analysis/

Diagnosis Decision 

Information System 

(CADDIS)

www.epa.gov/caddis

“Science is a way of trying not to 

fool yourself. The first principle is 

that you must not fool yourself—

and you are the easiest person to 

fool.” [Feynman 1964]

Photo courtesy USGS

Photo courtesy CT DEEP
Methodology 

The causal assessment framework follows the step-bystep process outlined in the diagram below. 

This framework:

• Provides scientifically defensible and 

reproducible evaluations

• Prevents biases and other lapses of logic

• Identifies causal relationships that are not 

immediately apparent

• Increases confidence that management 

efforts can improve biological condition.

What is CADDIS?

Causal Analysis/Diagnosis Decision Information System 

(CADDIS) is a web-based application that provides 

guidance, tools, and useful information for identifying 

causes of degradation of streams, rivers, and other 

bodies of water. The web site contains a detailed 

explanation of the causal assessment framework; 

a library of sources, stressors, and responses; case 

examples; data analysis tools; and a literature database.

Why was it developed 

and where can it help?

Thousands of bodies of water in 

the United States are biologically 

degraded, many by unknown 

causes. Causes must be 

identified so that management 

efforts can be implemented and 

waters can be restored. 

CADDIS can assist with Clean Water Act 

implementation and other management efforts, such as:

• More accurately identifying the causes of 

impairment in 305(b) reporting

• Accurately identifying the cause(s) of water 

quality standards violations for 303(d) listing 

and Total Maximum Daily Load (TMDL) 

calculations

• Prioritizing stressors for control, 

remediation, and restoration.

Why use CADDIS in TMDL 

development?

Before the TMDL process can be used to formulate 

appropriate management actions, the cause of 

impairment must be determined. TMDLs may require 

costly retrofits, thus the identified causes of impairment 

must be scientifically defensible. Defensibility requires 

knowledge of causal pathways by which stressors 

affect ecosystems, and understanding of symptoms and 

stressor-response relationships. CADDIS provides 

assessors with necessary knowledge to appropriately 

identify the pollutants causing the degradation.

CADDIS web site

www.epa.gov/caddis

The CADDIS web site contains information on 

all aspects of the causal assessment process. It is 

organized into five volumes:

Volume 1 explains 

the causal assessment 

framework.

Volume 2

describes common 

pollutants (nutrients, 

metals, herbicides, 

etc.) including their 

origins and effects. 

Relationships among sources, stressors, and biotic 

responses are described in conceptual diagrams. 

Volume 3 includes case studies and examples 

of techniques, methods, and tools. A recently added 

feature allows users to click on locations on a map to 

view details about case studies.

Volume 4 houses 

downloadable software, 

tools, and scripts for use 

in analysis of data.

Volume 5 contains 

a literature database 

(CADLit) and the 

Interactive Conceptual 

Diagram (ICD) Tool, 

which allows users 

to create conceptual 

diagrams for their causal 

assessments, and then 

link supporting literature 

to those diagrams.

Example of a conceptual diagram. 

Conceptual diagrams describe 

relationships among sources, 

stressors, and biotic responses 

useful for identifying potential 

causes.

Identify and Apportion Sources

Management Action: 

Eliminate or Control Sources, Monitor Results

Biological Condition 

Restored or Protected

Define the Case

List Candidate Causes

Evaluate Data from the Case

Evaluate Data from Elsewhere

Identify Probable Causes

Detect or Suspect 

Biological Impairment

As 

Necessary: 

Acquire 

Data and 

Iterate 

Process

Decisionmaker and 

Stakeholder 

Involvement