Association of State Wetland Managers - Protecting the Nation's Wetlands.

The Compleat Wetlander: Identifying Streams: East versus West

Streams are generally labeled perennial, intermittent or ephemeral.  The glossary of meteorology defines the terms this way:

ephemeral stream—A stream channel that carries water only during and immediately after periods of rainfall or snowmelt.
intermittent stream—A stream that carries water a considerable portion of the time, but that ceases to flow occasionally or seasonally because bed seepage and evapotranspiration exceed the available water supply.
perennial stream—A stream that contains water at all times except during extreme drought.

The U.S. Army Corps of Engineers description is similar but a little more detailed:

Ephemeral stream: An ephemeral stream has flowing water only during, and for a short duration after, precipitation events in a typical year. Ephemeral stream beds are located above the water table year-round. Groundwater is not a source of water for the stream. Runoff from rainfall is the primary source of water for stream flow.

Intermittent stream: An intermittent stream has flowing water during certain times of the year, when groundwater provides water for stream flow. During dry periods, intermittent streams may not have flowing water. Runoff from rainfall is a supplemental source of water for stream flow.

Perennial stream: A perennial stream has flowing water year-round during a typical year. The water table is located above the stream bed for most of the year. Groundwater is the primary source of water for stream flow. Runoff from rainfall is a supplemental source of water for stream flow.

Given these definitions it seems like it should be easy to distinguish one from the other in the field.

But it’s not.  Years ago the State of Wisconsin decided they were going to map all of the perennial streams in the state.  The problem was that while the state was trying to create the map, some of the streams changed from intermittent to perennial or perennial to intermittent in response to changes in surrounding land use and other factors.  That’s just the perennial/intermittent interface.  Upstream intermittent streams become ephemeral when they no longer come in contact with groundwater and it’s even more challenging to identify the transition from intermittent to ephemeral streams.  The presence of groundwater is not always easy to discover.

Furthermore, in addition to the definitions above, many states have legal definitions of intermittent streams and ephemeral streams that differ from the scientific definition. Then there are the court cases.

This means that, like wetland identification and delineation, stream identification and delineation in intermittent and ephemeral streams cannot rely on the presence of water. Scientific methods for identifying streams use observable flow indicators and place a heavy reliance on the presence of an ordinary high water mark (OHWM).

The ordinary high water mark (OHWM) is the point on the bank or shore up to which the presence and action of the water is so continuous as to leave a distinct mark either by erosion, destruction of terrestrial vegetation or other easily recognized characteristic.

In the East the ordinary high water event is created by a two year storm event.  This event creates “bankfull” condition in streams.  The stream fills up to the edge of the bank.  In the East precipitation is both regular and frequent leading to the creation of well-defined streams, where it is easy to identify the OHWM.  Also, in the East streams generally stay within their banks except during major storm/flood events when they overflow into the adjacent floodplain.

In the Arid West it is different.  To begin with the Corps identifies an ordinary event as a 5-10 years event in the West because rainfall is so infrequent compared to the East.  The lack of rainfall also leads to the absence of thick vegetation to cover the ground and roots that hold the soil in place.  Thus when an ordinary water event occurs there are no roots holding sand and gravel in place.  As a result everything moves around a lot more including the river channel.

Identifying the ordinary high water mark in the West requires finding the “active floodplain” because in a normal event the last thing that happens is the lowest part of the stream gets filled in with sand and gravel, and a new low flow channel is carved out nearby.  Then the whole process starts over again.  The active floodplain is the area where channels are carved and re-carved as the channel moves back and forth during ordinary events.  It is the equivalent of “bankfull” conditions in the East.

For information about stream identification in the West:

A Field Guide to the Identification of the Ordinary High Water Mark (OHWM) in the Arid West Region of the Western United States: A Delineation Manual Robert W. Lichvar and Shawn M. McColley August 2008

More information about the importance of ephemeral and intermittent streams in the West can be found at:

The Ecological and Hyrdological Signifigance of Ephemeral and Intermittent Streams in the ARaid and Semi-arid American Southwest

More on intermittent streams: Vanishing Veins of the Watershed

From Australia: Managing Flows for Ephemeral Streams

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