Environmental consultants and agency professionals in Tennessee rely on a process known as a Hydrologic Determination (HD) to identify the difference between a stream and a wet weather conveyance. How is it done? Why does it matter? Can anyone perform an HD?
WHY CLASSIFY STREAMS?
Most states use a classification system to determine a score that gives us some idea of how a stream is functioning. By maintaining stream health and biodiversity, we protect the quality of our own drinking water. With this understanding in mind, the Tennessee Department of Environment and Conservation (TDEC) has worked to define the difference between which watercourses need a permit to be altered and which do not. Much of our state's process was modeled after the North Carolina Department of Water Quality Classifications, to develop a scientific method for determining watercourse classifications based on relevant scientific disciplines. Two designations for watercourses were derived for Tennessee, streams and wet weather conveyances (WWC), in a process known as a Hydrologic Determination (HD).
HOW DO WE DO IT?
A data sheet (backed by a guidance document) is taken into the field to properly document the channel in question. The HD field form is split into conclusive, more obvious primary indicators and less obvious secondary indicators. If any of the following primary indicators of a WWC or stream are met, the investigation may stop.
There are four primary indicators that define a WWC:
The watercourse exists solely due to a process discharge;
A defined bed and bank are absent and the feature is dominated by upland vegetation;
The watercourse is dry anytime between February and April 15th; and
Daily precipitation records indicate the watercourse flows in direct response to rainfall.
The remaining five primary indicators suggest that the watercourse is a stream:
The presence of multiple populations of obligate lotic organisms;
Presence of fish;
Presence of naturally occurring ground water table connections (seeps and springs);
Flowing water in the channel (without recent rainfall); and
Evidence that the watercourse has been used as a supply for drinking water.
In some situations, these primary indicators are too difficult to assess, either because the watercourse is too wet, too dry, man-made, impacted, etc. Secondary indicators may then be used to inform the HD call. There are 28 secondary indicators concerning geomorphology, hydrology, and biology that provide a numerical score for the watercourse. Indicators such as sinuosity, macroinvertebrate assemblages, and the amount of leaf litter in the channel, help the assessor understand flow regimes within the channel.
If the watercourse is scored at a 19 or below, the channel in question is likely a wet weather conveyance. At 19 and above, it is more likely to have developed the characteristics of a stream. The numerical score is to be used as a guide rather than a definitive answer. For example, if the watercourse in question has been impacted recently, it may retain some core functionalities of a stream, but may not score a 19 or higher. Similarly, some smaller headwater habitats may look like a stream, but will not fall under the definition of a stream according to TDEC.
WE HAVE A SCORE. NOW WHAT?
In Tennessee, TDEC emphasizes the difference between WWCs and streams because alterations to WWCs do not require the submittal of an application or written authorization prior to impact activities, so long as they are considered minimal. These watercourses do not hold water long enough to affect downstream water quality. On the contrary, if the watercourse in question is considered a stream, an Aquatic Resource Alteration Permit (ARAP) must be submitted and authorized prior to development. This permitting process is much more restrictive and often authorizes the mitigation (replacement) of stream habitat, depending on the impact activity.
The US Army Corps of Engineers (USACE, sometimes referred to as, "the Corps") maintains authority over the Waters of the US and monitors stream impacts as well. The Corps determines the jurisdictional status of a watercourse based on its Ordinary High Water Mark (OHWM). Because the USACE is focused on this physical characteristic, some natural watercourses that do not exhibit the characteristics of a stream according to TDEC may qualify as a stream to the USACE. During the permitting process, both agencies will need to investigate the watercourse and future impacts to determine how it will be permitted for development.
WHO CAN PERFORM AN HD?
This HD guidance is currently used for every region of the state, which can make the determination process more difficult in very wet or very dry conditions. To increase the utility of the guidance in these situations and to standardize the HD process across private companies and government agencies alike, TDEC administers a mandatory three-day, $600 HD training class. Each qualified professional must also pass a written and field exam. Once certified, all Qualified Hydrologic Professionals (QHPs) must take a refresher course every three years. The course consists of a brief set of lectures and a field exam.
Circadian employs Qualified Hydrologic Professionals to conduct such evaluations. We have acquired over five years of experience delineating streams in North Dakota, Montana, Ohio, West Virginia, and Tennessee. Contact us to discuss your pre-development permitting determination or for more information about the process.