Summary #
Fluvial geomorphology (FGM) is the study of how natural stream channels form and function. It provides the scientific basis for natural stream channel restoration — returning relocated or damaged streams to stable, self-maintaining designs that support aquatic life and manage flood flows efficiently. FGM-based design is more cost-effective and ecologically sound than traditional engineered channel approaches.
What is Fluvial Geomorphology? #
Fluvial geomorphology (FGM) is the study of how natural stream channels form. This knowledge is applied to natural stream channel restoration after construction projects or urban expansion require streams to be relocated. The goal is to return streams to their natural character, reestablish floodplains and riparian buffer zones, limit livestock access, and use bioengineering to stabilize soils and reduce stream bank erosion.
Why Traditional Hydraulic Engineering Falls Short #
Previously, hydraulic engineering techniques designed relocated stream channels to handle extreme storm events — even the 100-year storm. The result was smooth, straight, high-velocity channels requiring expensive concrete and rip-rap stabilization. These channels were ecologically barren — high velocity and smooth surfaces left no habitat for algae, macroinvertebrates, or other aquatic organisms. Despite high flow rates, these channels paradoxically required constant dredging to remove accumulated sediment.
The FGM Approach #
Years of detailed study of naturally stable streams led to the FGM approach. Natural stable streams are designed by nature to handle frequent one- and two-year storm events — the typical thunderstorm — rather than rare catastrophic floods. These streams maintain themselves through a balance of sediment deposition and natural bottom scouring. When major flood events occur, floodplains absorb the overflow, reducing velocity and capturing sediment.
When a stream must be relocated using FGM principles, it is reconstructed to mimic its natural form as closely as possible. If the stream passes under a bridge, culverts or flood flow pipes are placed at the height of the full-running stream channel. Overflow is channeled into the floodplain, where surrounding vegetation slows flow and prevents excess sedimentation.
Benefits of FGM #
FGM-based stream design offers significant advantages over traditional engineered channels. Bridge construction is simpler, flood flow pipe requirements are reduced, and span area is minimized. Less money is needed for concrete and rip-rap. Aquatic habitat is preserved or enhanced. Construction aesthetics are improved.
For in-depth information on FGM stream classification and design, see Applied River Morphology by David Rosgen.
Pennsylvania’s Keystone Stream Team Natural Stream Channel Design Guidelines: http://www.keystonestreamteam.org/kst_guidelines.htm
General FGM overview — Progress and the Environment from Skelly and Loy, Inc.: http://amrclearinghouse.org/Sub/WATERSHEDbasics/skelly_and_loy.pdf
Related Pages #
- Riparian Buffers (Watershed Tools — Watershed 101)
- Stewardship (Watershed Tools — Watershed Health)
- Land Use (Watershed Tools — Watershed Health)
- Evolution of the Watershed (Watershed Tools — Watershed Health)
Source and Last Reviewed #
Sources: http://www.keystonestreamteam.org/kst_guidelines.htm | http://amrclearinghouse.org/Sub/WATERSHEDbasics/skelly_and_loy.pdf
Last reviewed: 2026-03 | Links may require verification — originally published pre-2010.
Tags: assessment, monitoring, practitioner, education
