Indicators of

Hydrologic 

Alteration

By Steve Swanson, River Modeling

Specialist, BLM, National Science

and Technology Center

Hydrologists of the Bureau of

Land Management have long been

challenged to identify and record

anthropogenic effects on the natural flow regimes of streams and

rivers on public lands. Increased

public concern and legislation over

the past 20–30 years have resulted

in heightened scrutiny of the consequences of human-caused alterations to these natural systems.

The historical tools available to

answer the question “How much

hydrologic alteration is too much

for a riverine system?” have not

been adequate. Historical methods may focus on specific portions

of the ecosystem, such as aquatic

biology or riparian functionality,

rather than the system as a whole.

Another limitation involves the

fundamental differences in language and analytical methods

that exist between hydrologists

and ecologists or biologists.

Hydrologists tend to be concerned with the statistics of

monthly and annual averages and

the frequencies of floods and flows.

Ecologists and biologists are often

concerned with flow durations,

low-flow extremes, and the rates at

which water levels rise and fall.

Hydrologic Parameters Indicate Anthropogenic Effects

The Nature Conservancy has

developed an approach, led by

Brian Richter, in which the gaps in

communication and methods are

bridged. This approach, termed

Indicators of Hydrologic Alteration (IHA), is a suite of 33

hydrologic parameters that are

ecologically meaningful and serve

as sensitive indicators of anthropogenic effects on riverine systems. These parameters and their

potential influences are shown in

the Table.

Some of these parameters are difficult or impossible to calculate by

using standard spreadsheet or

statistical software. The National

Science and Technology Center

(NSTC), Denver, Colorado, uses

an IHA software package (Smythe

Scientific Software) that calculates

the entire suite of parameters by

using daily streamflow data

obtained from the U.S. Geological

Survey. The program also plots the

results and provides two types of

statistical analysis and three

methods of parameter analysis and

interpretation:

Statistical Analysis

• Nonparametric Statistics: The

highly skewed nature of most

hydrological data sets dictates

this method in many situations.

The software uses the median as

a measure of central tendency

for the IHA parameters

• Parametric Statistics: This type

of analysis assumes a normal

distribution, with the mean as a

measure of central tendency for

IHA parameters.

Parameter Analysis

• IHA Analysis: This analysis

provides a pre-impact versus

post-impact comparison. A

“scorecard” table and graphs

of each IHA parameter are

produced to quantify changes

in each parameter between

the pre- and post-impact flow

regimes.

• Range of Variability Analysis: In

this analysis, the user defines

three categories that divide the

range of pre-impact data values.

Delineation into three equal

categories, for example, would

result in parameters less than or

equal to the 33rd percentile,

parameters between the 33rd

and 67th percentile, and parameters greater than or equal to

the the 67th percentile. The

program then calculates the

expected frequency at which the

post-impact values should

occur in each category. A comparison of these two data sets

provides a pre- and post-impact

measure of the hydrologic alteration between flow regimes.

• Trend Analysis: A complete

graphical historical analysis of

the IHA parameters is produced, together with a linear

regression analysis of the data.

Staff of the NSTC are presently

using the Indicators of Hydrologic

Alteration approach on a project

in southern Arizona. This approach

also has significant application to

other locations within the Bureau. 

Contact

Steve Swanson, National Science

and Technology Center,

Denver Federal Center, 

Building 50, P.O. Box 25047

Denver, CO 80225-0047

phone: 303-236-8693

fax: 303-236-3508

email: Steve_Swanson@blm.gov

NO. 58 DATE 09/09/02

Range

Hydrology
Resource Note 58

page 2

Range

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Science Editor - Brian St. George (Brian_St_George@blm.gov)

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Thank you for your interest in Resource NoteS.

Table. Indicators of Hydrologic Alteration.

IHA group

Magnitude of monthly water 

conditions

Magnitude and duration of annual

extreme water conditions (mean daily

flow)

Timing of annual extreme water 

conditions

Frequency and duration of high and

low pulses

Rate and frequency of water condition

changes

Hydrologic parameters

Mean value for each calendar 

month (12 parameters)

1. Annual 1-day minima

2. Annual 3-day minima

3. Annual 7-day minima 

4. Annual 30-day minima

5. Annual 90-day minima

6. Annual 1-day maxima

7. Annual 3-day maxima

8. Annual 7-day maxima

9. Annual 30-day maxima

10. Annual 90-day maxima

11. Number of zero-flow days

12. 7-day minima/mean for year

1. Julian date of each annual 1-day

maxima

2. Julian date of each annual 1-day

minima

1. Number of low pulses within each

year

2. Mean duration of low pulses each

year

3. Number of high pulses within each

year

4. Mean duration of high pulses each

year

1. Means of all positive differences

between consecutive daily values

2. Means of all negative differences

between consecutive daily values

3. Number of hydrologic reversals

Ecosystem influences

1. Availability of habitat for aquatic

organisms

2. Availability of soil moisture for

plants

3. Availability of water

4. Reliability of water supplies for

wildlife

5. Effects of water temperature and

dissolved oxygen

1. Balance of competitive and stresstolerant organisms

2. Creation of sites for plant 

colonization

3. Structure of river channel morphology and physical habitat conditions

4. Soil moisture stress in plants

5. Dehydration in wildlife

6. Duration of stressful conditions

7. Distribution of plant communities

1. Predictability and avoidability of

stress for organisms

2. Spawning cues for migratory fish

1. Frequency and magnitude of soil

moisture stress for plants

2. Availability of floodplain habitat

for aquatic organisms

3. Effects of bedload transport and

channel sediment distribution, and

duration of substrate disturbance

1. Drought stress on plants

2. Desiccation stress on low-mobility

streamedge organisms