How many years of data does this trend use?

5 years of EPA Water Quality Portal samples, 2020–2024. Each metric (clarity, phosphorus, chlorophyll-a) is summarized as the annual median of all samples for that year, then trended via linear regression. Five-year windows are short for climate-scale claims — treat short-term trends as directional, not definitive.

What could cause a water quality change at Agnes Lake?

Common drivers include shoreline development, agricultural runoff, invasive species (zebra mussels notably increase clarity), road salt, climate warming, watershed land-use change, and changes in wastewater treatment upstream. A short-window trend at Agnes Lake could reflect any of these or normal year-to-year variability.

Agnes Lake Water Quality Trend

St. Louis County, Minnesota · 2020–2024 · 5 years of data

Agnes Lake water quality has been declining over the 5-year window from 2020–2024, based on EPA Water Quality Portal samples. The lake holds an overall Grade D today.

Water Clarity (Secchi)

→ Stable

2020–20240.00 m/yr

Phosphorus

↓ Declining

2020–2024+2.55 µg/L/yr

Chlorophyll-a

Insufficient data (0 years, need 4+).

How to read this

Clarity (Secchi depth): deeper visibility is better. An upward line means the water is getting clearer.
Phosphorus & Chlorophyll-a: lower is better. A downward line means fewer nutrients fueling algae growth.
Year-to-year variability is normal. Weather, sample timing, and short-window data all add noise. Look at direction, not single-year jumps.

Methodology

Each metric is calculated from EPA Water Quality Portal samples. Annual values are the median of all samples taken that year. The trend direction comes from a linear regression of those annual medians; we classify it as improving, declining, or stable when the change-per-year is below 2% of the long-run mean. The current cache spans roughly 2020–2024, which is a short window for climate-scale claims. Use this page to spot direction and as a launch point — link out to the EPA WQP for the raw record before citing as evidence.

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