Inundation Frequency analysis with application to development of alligator gar HSI

GCPO LCC Webinar, 5-5-2014

Floodplain Inundation Frequency Analysis and its Application to Alligator Gar Spawning Habitat in the Gulf Coastal Plains & Ozarks (GCPO) region

Link to recorded webinar:

Presenter: Yvonne Allen - Aquatic Habitat Analyst, GCPO LCC 

Major points covered

  • Aquatic assessment of the GCPO region using Inundation Frequency (IF) analysis
  • Overview of IF estimation methodology
  • Virtual tour of IF in the GCPO region
  • Relating IF GIS analysis to ground conditions
  • Using IF to help identify alligator gar spawning habitat

Presentation Summary 

Under the GCPO LCC Integrated Science Agenda, aquatic and terrestrial habitat assessments are a first step toward an overall conservation design.  The assessments will need information on the amount, configuration and condition of habitat.  Inundation dynamics are a primary driver of floodplain habitat configuration and function.  Defining floodplain inundation is key to assessing habitat for many species, both aquatic and terrestrial.

The inundation frequency (IF) analysis uses 30m pixel resolution landsat imagery; includes all landsat scenes that cover the GCPO region.  Because landsat images have been repeated every 16 days from 1984 to 2011, they provide views of a wide range of conditions from very low to very high water.

Suitable landsat images over time are analyzed to classify extent and frequency of inundation.  This image analysis then linked to river gage records.

Virtual tour of IF in the five GCPO subgeographies (West Gulf Coastal Plain, East Gulf Coastal Plain, Mississippi Alluvial Valley, Ozark Highlands and Gulf Coast)

Using IF to identify alligator gar (AG) spawning habitat

One of the most limiting factors for AG success is spawning habitat; several variables and thresholds identified by AG biologists that could limit spawning have to do with floodplain availability, access and duration.

How to apply relative measure of IF to something more meaningful:

  1. determine the watershed of interest
  2. representative of which gaging station? - relate to a record that has meaning for that geography (chose Natchez for the AG study at St Catherine’s Creek NWR, or SCC NWR)
  3. evaluate representativeness of IF

Created a distribution of river gage data compared to landsat image data on the same dates:

  • AG spawning occurs Apr -Jun along the lower Mississippi River
  • Identified river stage/landsat threshold where can expect to see a location inundated 50% of the time during spawning season and relate that back to the mapping product. 

Address water temperature and physical structure.  The mainstem Mississippi river in spring is very cold compared with temperatures that may be found on the adjacent floodplain- 59 degrees vs 74 degrees on one observed date in 2014.


  • look at temperature difference from river
  • identify the locations with thermal advantage in a process similar to IF
  • stack the average temperature difference data over the same 24 Landsat images through time

St. Catherine’s Creek NWR is very warm compared to river and other places along the river.

Physical structure:

  • Identify areas with low vegetation and low to no canopy.
  • Used USDA cropland data layer to identify vegetation type (most recent data, there are lots of data sources).

Put the IF, temperature analysis and vegetation data together into a Habitat Suitability Index for the lower Mississippi River for alligator gar spawning.  The HSI for the lower Mississippi River corridor is available on the GCPO LCC Conservation Planning Atlas.  It identifies best existing areas for spawning and also areas where changes in management could potentially improve conditions for spawning.

Gar telemetry over 3 years at SCCNWR guided development of the HSI, and observations in 2014 have definitely found gar spawning in areas designated as most suitable by the HSI.