Floodplain hydrology is the degree to which water accesses and hydrates the land surrounding the channel. The complexity and structure of riparian habitats are determined by the timing, extent, duration, and depth of floodplain inundation. Floodplain connection is important for many reasons. Floodplain flows recharge groundwater and raise the local water table, promoting healthy riparian vegetation and supporting the exchange of water, sediment, organic matter, nutrients, and organisms between the river, floodplain, and alluvial aquifer. Groundwater recharges the stream channel during times of low flow, helping maintain baseflows and moderate water temperature, both factors that are critical for aquatic habitat.
Functional floodplains moderate the downstream effects of flood flows—flows carrying potentially destructive energy to downstream communities. During extreme flood events or debris flow events, high flows spread across floodplains where they are slowed by vegetation, retained temporarily as surface storage, and released over time through groundwater infiltration. Not all rivers have the capacity to regulate floods: those in canyon environments, for example, have little natural capacity.
Floodplain connectivity can often be compromised by hydrological modifications (changes to the natural hydrograph due to dams and diversions), channel alteration (such as channelization, bank/channel armoring, enlargement, and entrenchment), and land uses in the floodplain (including levees, drainage ditches, development, transportation corridors, and floodplain fill).
Analysis of floodplain hydrology should begin with a review of all available related data. Decisions regarding assessment level and data analysis should be made based on SMP goals.
Inundation extent at different flows/return intervals
Maps of structural features that limit floodplain connection
Maps showing floodplain fragmentation by transportation and development
Estimates of historical and current floodplain extent
Riparian vegetation maps
Potential Data Sources
USGS, Colorado Department of Water Resources, and other stream gauges
USGS 3D Elevation Program (3DEP, formerly National Elevation Dataset (NED))
Colorado LiDAR imagery repository
1D/2D hydraulic models (e.g., HEC-RAS)
Remote desktop surveys using aerial imagery to identify features limiting floodplain connection
Remote desktop surveys using aerial imagery to estimate historical floodplain extent
Field visits to identify flood indicators
Field visits during high-flow conditions (spring runoff)
Riparian vegetation surveys
Interviews with local residents
Tips for Success
Survey data and digital elevation models (DEM) can be plugged into hydraulic modeling software such as HEC-RAS to calculate 2-year flow, 10-year flow, and 100-year flow (or other return intervals) floodplain extents for detailed inundation modeling.
Topographic estimation can be used to map the extent of saturation/inundation of the floodplain at 2-year, 10-year, and 100-year (or other) flow return intervals.
Aerial imagery can help identify levees, artificially high banks, or other structural features that limit flood extent. It can also be used to assess floodplain development, including conversion of riparian area to urban and rural land uses.
Riparian vegetation surveys or mapping can be used to determine inundation extent based on type and species of vegetation present. Rapid field assessments (particularly during high-flow conditions) can utilize expert knowledge of geomorphological processes and riparian ecology to determine the extent to which flow regime modification or structural impediments limit connections between the channel and floodplain.
In populated areas with development, floodplains are necessarily controlled to protect people and property. However, in those situations, management options to increase floodplain connectivity can include parks, common areas, and trail systems that can be flooded. Where no development exists, floodplains can be protected through conservation easements.
Crystal River Management Plan
Assessment Level: Level 3
Evaluations of floodplain hydrology for the Crystal River Management Plan relied on reviews of aerial imagery, rapid field assessments, and hydraulic simulation modeling. Aerial imagery reviews revealed locations of floodplain development, and field visits evaluated the extent to which development in the floodplain, structural barriers, and flow regime modifications limit floodplain access. The Ecological Decision Support System (EcoDSS) developed to understand historical and current flow regime characteristics along the Crystal River simulated two-dimensional channel flow along the lower Crystal River across a range of hydrological conditions (e.g., average flow, moderate flood, extreme flood) and compared current and natural (no surface water diversions) inundated floodplain areas.
All three assessment techniques revealed small pockets of considerable impairment to floodplain hydrology, generally attributable to current and historical transportation corridors near the river bisecting the accessible floodplain, fragmenting off-channel habitat, reducing riparian zone size, and artificially constricting unconfined reaches.
Yampa River Health Assessment and Streamflow Management Plan
Assessment Level: Level 3
To assess floodplain hydrology within the five study areas through the City of Steamboat Springs, the Yampa River Health Assessment used HEC-RAS inundation modeling to calculate inundation extents at a range of flows (high (1-2 years), medium (5-10 years), and low (50-100 years) frequency floodplains); topographic survey estimation to map the extent of floodplain saturation/inundation at a range of flows; review of aerial imagery to map structural features that limit floodplain extent; review of aerial imagery to map estimated historical (natural) floodplain extent; field identification of flood indicators; field observation of all reaches during runoff conditions; and riparian vegetation mapping.
Through these lines of evidence, the Yampa River Health Assessment found reduced floodplain connection to be one of the most critically impaired aspects of ecosystem function throughout the study area. On most reaches, the floodplain was only inundated during exceptional runoff years. Channel impacts and land uses in the floodplain that contribute to this issue include roads and railroad beds that act as levees, bridges that constrain flow passage, and urban encroachment.