Thole Lake Water Quality Trend

Reading Thole Lake's trajectory

EPA samples over 6 years show Thole Lake's water quality declining. Per-metric: clarity is trending toward improvement; phosphorus is trending toward degradation; chlorophyll-a is trending toward degradation. Declining lakes often hit symptom thresholds (visible algal blooms, fish-kills, swim advisories) only several years into the trend; the multi-year metric direction is an earlier and more reliable signal than the visible state of the water on any given day.

A 6-year sample window is enough to surface directional trends but short enough that one or two unusual weather years can still influence the read. Use the trend as a signal, but combine it with the most recent absolute Grade rather than relying on direction alone. The state environmental agency's lake-specific monitoring reports often add context that a five-year trend line by itself can't carry.

Clarity (Secchi depth), phosphorus, and chlorophyll-a are the three metrics most consistently sampled across years and most useful as trend indicators. Clarity captures the visible signal — how deep you can see into the water. Phosphorus is the primary nutrient driving algal growth in most lakes. Chlorophyll-a measures algal biomass directly. Together they describe the same underlying water-quality state from three angles; when all three agree on direction, the trend signal is strong.

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 2020–2025, 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.