The phosphorus data shown here represents an estimate of total Vermont loads of phosphorus, based on estimated loads from major Vermont tributaries to Lake Champlain. Total gaged areas for tributaries located primarily in VT are adjusted by the proportion of gaged area relative to total VT watershed area in the Lake Champlain drainage to estimate total VT loads. This number is an approximation, as gaged areas may not be a perfect analogue to un-gaged areas, particularly in the direct Champlain drainage area. As a result, annual phosphorus loading patterns closely follow annual stream flow patterns. The target load of 418 metric tons total phosphorus represents the maximum amount of phosphorus the lake can receive each year, as specified by the Phosphorus Total Maximum Daily Loads (TMDLs) for Vermont Segments of Lake Champlain, and continue to meet water quality standards; however, actual annual loads will vary significantly with changes in streamflow, and wet years will frequently have 2-3 times the estimated load as dry years. As a result, the lower loading numbers in 2020-2022 relative to 2018 and 2019 primarily reflects that these were relatively dry years, rather than reductions due to changes in nutrient management, and the high precipitation in 2023 will likely associated with considerably higher loading. With the passage of the Vermont Clean Water Act (Act 64) in 2015, we now have additional permitting and funding tools to further reduce phosphorus loads to our rivers, streams, and lakes. Decreased loading should be measurable at a local level (individual smaller rivers and streams) as implementation progresses, however it is likely to take many years to show substantial progress in the larger Champlain tributaries and the lake itself. The DEC utilizes additional metrics to evaluate load reductions over time (see our annual RBA report for more information). 

The target load of 418 metric tons total phosphorus represents the maximum amount of phosphorus the lake can receive each year, as specified by the Phosphorus Total Maximum Daily Loads (TMDLs) for Vermont Segments of Lake Champlain, and continue to meet water quality

In this update, loads have been calculated for calendar years, whereas in previous updates they have been calculated for water year (Oct 1 – Sept 30).  This has resulted in some revision of previously reported values for 2018, 2019, and 2020. Loads were calculated on the basis of USGS tributary mointoring gages on major tributaries to Lake Champlain in the VT portion of the basin, coupled with tributary monitoring data collected by the Lake Champlain Long-Term Tributary Monitoring Program. Daily concentration estimates are calculated using the WRTDS model in the EGRET R package maintained by the USGS. Default smoothing values were used for model parameters, and Kalman filtering was applied after calculation of the model to improve accuracy of modeled concentrations.