Hydromodification: Parameters for Mitigation Requirements

Focus Area

Water Quality/Wetlands


Natural Resources







Research Idea Scope

Need for the Research

Highways can contribute to hydrologic changes with subsequent degradation of streams. Permits and regulations are beginning to require management of stormwater to minimize adverse hydromodification, but there are no consistent standards for determining when a water body needs hydrologic protection or setting a range of flows that should be managed.

Scope or Research

The research would focus on two aspects of hydromodification mitigation. The first goal is to develop criteria for differentiating between water bodies that are vulnerable to hydrologic changes caused by added impervious surfaces, and those that are not. The second goal is to determine the range of flows that need to be controlled to protect stream processes.

While most would agree that large rivers such as the Mississippi, the Columbia and the Willamette are not affected hydrologically by development, the low end of the range of river flows is not established. The State of Washington has developed detailed criteria, but that is an exception. Those criteria still need to be evaluated for general applicability across different regions. The research would consist of a) gathering criteria that may be in place in the various states and their rationales, b) conducting a critical review of hydrologic and geomorphic literature pertinent to this issue, and c) developing criteria and guidance based on the first two steps.

There are no consistent criteria established for the range of flows that need to be controlled to protect stream integrity. Oregon and Washington, for example, have different endpoints for flow control, despite having similar climates. Oregon’s criteria are based on informed geomorphic opinion, but the state has not undergone rigorous review for general applicability. The research would consist of a) a review of existing criteria and their rationales, b) critical review of hydrologic, geomorphic and biologic literature pertinent to this issue, particularly concerning validity of concepts used to identify endpoints and regional applicability, and c) development of guidance for criteria based on the first two steps.


The proposed U.S. EPA rulemaking will most likely require hydromodification mitigation for highways. The research should be started now to coincide with the release of the final EPA rule in 2012.

Urgency and Payoff

The proposed U.S. EPA stormwater rulemaking will likely require DOTs to mitigate for hydromodification impacts. By providing consistent, scientifically supportable criteria for when hydrologic mitigation is appropriate and for the range of flows that should be controlled, rational rulemaking would be supported, and the rules would provide protection to vulnerable water bodies while avoiding requirements that expend resources for no discernable benefit.

Suggested By

Center for Environmental Excellence, Stormwater Community of Practice