Assess Conditions and Identify Risks

Assess Conditions and Identify Risks

SMP Nuts and Bolts

Assess Biological, Hydrological, and Geomorphological Conditions

Fluvial Hazard Zone (FHZ) Mapping

A Fluvial Hazard Zone (FHZ) is defined as the area a stream has occupied in recent history, may occupy, or may physically influence as it stores and transports water, sediment, and debris. The process of mapping the fluvial hazard zone results in an improved understanding of the interactions between a watershed and its stream corridors, the major sources and sinks of sediment and large wood in the watershed, and stream- and flood-related hazards beyond those identified by traditional floodplain mapping. FHZ mapping also highlights the human and ecological interactions that influence the character and function of the stream corridor, and that have contributed to its current condition, and possible future trajectory. This exercise can prove extremely useful for the Stream Management Planning process because the FHZ mapping products paint a picture of the story of a river system over time, laying a foundation for developing holistic stream management strategies, restoration projects, and conservation priorities. While typical river health assessments that are undertaken as part of the “Assess Conditions and Identify Risks” planning phase characterize the current conditions of the river corridor, FHZ mapping looks at both the past and future of the river corridors to help tell the story of how the rivers are constantly changing and what upcoming changes could be expected.

Stream health assessments and Fluvial Hazard Zone mapping are complementary studies that together provide useful information to decision-makers in stream management planning efforts because many of the stressors that impair stream health are caused by actions taken to alter stream corridors for a variety of reasons (e.g., transportation infrastructure such as roadways, railroads, and crossings; water diversions and water storage reservoirs; suburban/rural residential and commercial developments; and flood control structures such as riprap, levees, and berms). Anticipating these hazards when making decisions about land-use planning, future development, conservation areas, and restoration projects is a good way to protect stream health and valuable ecosystem services while simultaneously reducing risk. This is the path to building healthy resilient communities.

More information on fluvial hazard zone mapping is available on the Colorado Water Conservation Board’s FHZ website, along with a helpful introductory video. The website includes links to the CWCB’s FHZ delineation protocol, links to overview fact sheets, and community planning and regulatory support resources.

Examples

Yampa Basin

Yampa River Basin

In early 2022, a FHZ mapping project and delineation was completed for the Yampa River Integrated Water Management Plan (IWMP) for approximately 35 miles of the Elk and Yampa Rivers. The project was recommended by the IWMP Riparian Work Group and sought to identify areas along and adjacent to the Elk and Yampa Rivers that are susceptible to the natural processes of erosion, sediment/debris deposition, and change to the location and configuration of active river channels. The project also identifies possible actions to enhance stream function and absorb or reduce the impacts of hazards, including land and water protection; floodplain and stream rehabilitation, reconnection, and restoration; and infrastructure considerations and retrofits. The project and its work products are described and presented in this Story Map.

Arkansas River

Upper Arkansas River and Tributaries

One of the first efforts to incorporate FHZ mapping into a stream health assessment was completed on a short 1.2 mile section of the South Arkansas River in 2020 as a component of the forthcoming Upper Arkansas Watershed Resiliency Plan. The stream health assessment and fluvial hazard zone mapping studies were conducted simultaneously to identify major stressors and causes of stream health impairment, identify flood- and stream-related hazards, and develop planning ideas for specific conservation and restoration strategies. The planning concepts identified in the report aim to increase overall resiliency and mitigate the dominant stressors of past channelization, historical land uses, wood removal, and beaver extirpation by mimicking, promoting, and sustaining the natural processes that have been disrupted by these stressors.

Assess Conditions and Identify Risks

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