Horse Creek Outlet Lake Water Quality Trend

Reading Horse Creek Outlet Lake's trajectory

EPA samples over 4 years show Horse Creek Outlet Lake's water quality declining. Per-metric: 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.

The 4-year sample window is short — long enough to flag possible directional shifts but not long enough to be confident the trend is structural rather than weather-driven. A single wet year, a single dry year, or a recent change in sampling location can pull a short-window trend in either direction. Treat the direction as suggestive rather than definitive; the absolute Grade and §303(d) listing status carry more weight than the trend over this period.

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 2020–2024, 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.