Flood Plains and River Terraces

Rivers are dynamic systems. River channels can migrate laterally as a result of bank erosion and deposition, and vertically as a result of bed aggradation or degradation. Floodplains, terraces, and other features are formed by these processes, and are therefore essentially part of the river system.

When a river channel reaches its maximum capacity, often during times of heavy rain or snow melt, water overflows the river’s streambanks and floods into nearby areas that would otherwise be dry. This is especially true when water is delivered at a rate faster than the associated soils can absorb it. Floods also occur when a dam or water impoundment gives way and large amounts of water are released suddenly. For the most part, flooding is a natural process that supports channel maintenance, ecological processes, and riparian vegetation. Nevertheless, floods can cause severe human impacts and therefore must be among resource planning considerations.

Within the SEUALG region, and the Colorado Plateau in general, flooding most often occurs from two distinct event types: (1) spring runoff from melting snowpack at high elevations (both local and regional), and (2) summer rainstorms [1]. While either event can trigger flooding, the dynamics of each are different. Snowmelt is a relatively predictable occurrence dependant on the amounts of winter snowpack and rising spring temperatures. Large accumulations of snowpack melting in spring contributes to some localized flooding, but more commonly flooding happens along the region’s larger rivers, including the Green, Colorado, San Juan, Dolores, San Juan and San Rafael. In contrast, summer cloudburst events, especially those driven by monsoonal moisture, cause sporadic and localized flooding events on otherwise dry washes and canyons. Both kinds of events can have profound impacts on the communities within the SEUALG area [2].  

Planning for major flood events should occur at all levels of government. At the federal level, the Federal Emergency Management Agency (FEMA) provides flood data that classifies areas based on their different flood hazards through the National Flood Hazard Layer (NFHL) and National Flood Insurance Program (NFIP). This enables community officials, emergency responders, and the general public to be informed and plan accordingly to avoid or reduce impacts from floods, guide development, and reduce risk of floods by excluding areas shown as flood hazard areas. Within the SEUALG region, only Carbon County and the Moab area are digitally mapped by NFHL. Recent mapping along Pack Creek (near Moab) is currently being evaluated by FEMA. Many of the remaining communities are covered by the NFIP, which uses paper maps to assess floodplain locations. A list of communities participating in the National Flood Program can be found here.

County and local planning efforts include the establishment of special zoning regulations in flood-prone areas.  However, these are not commonly used in the region.

Major economic considerations for floodplains are higher development costs to mitigate flood risks. Costs include earthen fill to raise building footprints above flood elevations and other flood-control structures on private lands. Flood-control costs may also be passed on to municipal and county governments during flood emergencies.

Another economic consideration is the cost of floodplain insurance to homeowners. Development in areas subject to floods should meet additional flood-proofing requirements. Laws and regulations regarding floodplain management usually vary between communities.

Floods are the leading cause of natural disaster deaths worldwide, and flooding has contributed to more than a dozen local deaths [2]. Floods also have the potential to cause significant financial impacts in the form of severe damage to structures, transportation systems, and other infrastructure.   

Wildfire is a secondary cause of flooding because when vegetation is burned, soils are exposed to erosion. Debris flows below fire scars is a considerable risk until vegetation is reestablished. Planning for revegetation through seeding and other mitigation efforts after fires should be addressed in resources management documents.