What is the National Lakes Assessment?
The National Lakes Assessment (NLA) is designed to provide statistically valid national and regional estimates of the condition of lakes. It uses a probability-based sampling design to represent the condition of all lakes across the coterminous United States. The survey is a collaborative effort between EPA, states, tribes, federal agencies, and other organizations.
The NLA is designed to answer the following questions about lakes across the United States.
- What is the current biological, chemical, physical and recreational condition of lakes?
- What is the extent of degradation among lakes?
- Is degradation widespread (e.g., national) or localized (e.g., regional)?
- Is the condition of lakes getting better, worse, or staying the same over time?
- Which environmental stressors are most associated with degraded biological condition in lakes?
NLA field season sampling is conducted every five years. Previous field seasons were conducted in 2007 and 2012.
The sampling design for the NLA is a probability-based network which provides statistically-valid estimates of the condition of all lakes with known confidence. Lakes are selected randomly using a statistical survey design to represent the population of lakes in their ecological region – the geo‐graphic area in which climate, ecological features, and plant and animal communities are similar.
The NLA sampling is comprised of natural lakes, ponds, and reservoirs across the lower 48 states. Starting with the NLA2012, to be included in the survey, a water body had to be a natural or man-made freshwater lake, pond or reservoir, greater than 2.47 acres (1 hectares), at least 3.3 feet (1 meter) deep, and with a minimum quarter acre (0.1 hectare) of open water. Lakes had a minimum retention time of 1 week. The Great Lakes and the Great Salt Lake were not included in the survey, nor were commercial treatment and/or disposal ponds, brackish lakes, or ephemeral lakes. The NLA 2007 assessed only those lakes greater than 10 acres (4 hectares) in size.
Lake ecosystems are dynamic and indicators selected to characterize them should represent their varied aspects. For the NLA 2012, a suite of chemical, physical and biological indicators were chosen to assess biological integrity, trophic state, recreational suitability, and key stressors affecting the biological quality of lakes (see table below). Although there are many more indicators and/or stressors that affect lakes, NLA analysts believe these to be among the most representative at a national scale.
For general descriptions of the indicators used for NLA as well as those used in the coastal survey (NCCA), the rivers and streams survey (NRSA), and the wetlands survey, please visit the Indicators used in the National Aquatic Resource Surveys page.