Horsetooth Water Quality Study

Horsetooth Reservoir is one of the two primary Northeastern Colorado reservoirs that store water from the Colorado-Big Thompson and Windy Gap projects for delivery to water users. The reservoir’s water goes directly into treatment plants for the Tri-Districts and the cities of Fort Collins and Greeley, which produce drinking water for more than 300,000 people.

Fort Collins, Greeley, the Tri-Districts, the U.S. Bureau of Reclamation and Northern Water conducted a collaborative study that monitored the reservoir’s middle thermal layer (metalimnion) in 2009, and in 2012 developed  a water quality model to serve as a tool for understanding the causes of low dissolved oxygen and the impacts of watershed and operational changes on water quality.

Water Quality Concerns
Horsetooth Reservoir’s water quality, which has been the subject of concerns related to aquatic life and drinking water treatment, has been monitored for more than 20 years.
Aquatic Life
Aquatic life need dissolved oxygen to survive. In Horsetooth Reservoir, dissolved oxygen in the metalimnion decreases over the summer. When the dissolved oxygen levels drop too low in the metalimnion (below 6 milligrams/Liter), this becomes a concern if fish are taking refuge in this layer. 

See full-size graph.
Temperature Profiles
Temperature Profiles measured by Northern Water in Horsetooth Reservoir in August 2010 showing typical thermal layers (stratification) that develop over the summer and into the fall.

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Dissolved Oxygen
Dissolved oxygen measured by Northern Water in Horsetooth Reservoir in August 2010 showing typical development of low dissolved oygen levels in the metalimnion and at the reservoir bottom.

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Hansen Feeder Canal Water
Hansen Feeder Canal water containing organic matter flows into Horsetooth Reservoir in the metlalimnion where microorganisms break down organic matter and, in the process, consume oxygen.

Drinking Water

Drinking water treatment issues include: 

  • Manganese is released from the bottom sediments as dissolved oxygen is depleted at the reservoir bottom in late summer and fall. Manganese can stain plumbing fixtures and laundry if not removed at water treatment plants.
  • Geosmin, a compound with an earthy taste and odor produced by some blue-green algae, can occur at high levels and is difficult to remove during treatment. 
  • Total organic carbon (TOC) concentrations, a measure of organic matter, have increased in the last several years. Organic matter occurs naturally from sources such as watersheds’ vegetation and soils, and organisms (including algae) living in reservoirs. It reacts with chlorine that is added at the water treatment plants, producing regulated carcinogenic compounds. 
Water Quality Model Findings 

Causes of Low Dissolved Oxygen

  • The primary cause of low dissolved oxygen in the metalimnion is organic matter flowing in from the Hansen Feeder Canal. Microorganisms degrade, or break down, inflowing organic matter within the reservoir’s metalimnion and consume oxygen in the process.
  • Like most deep reservoirs, low dissolved oxygen concentrations at the bottom of Horsetooth Reservoir result from the degradation of settled organic matter by microorganisms when the reservoir is naturally divided into thermal layers. In summer and into fall, when a reservoir has distinct thermal layers, the isolated and colder bottom layer has no opportunity for oxygen replenishment, so dissolved oxygen levels continue to drop until late fall when a reservoir mixes, eliminating thermal layering or stratification. The longer the duration of thermal layering before mixing, the lower the amount of dissolved oxygen at the bottom.  

Horsetooth Reservoir

Horsetooth Water Quality Study

Impact of Reservoir Inflow Water Quality
  • Organic matter that flows into Horsetooth Reservoir from the Hansen Feeder Canal is a complex mixture of compounds. Some compounds are easier for microorganisms in the reservoir to degrade. The levels of dissolved oxygen in the reservoir during stratification and the organic matter (TOC) concentrations in the outflow at Soldier Canyon Dam are impacted by the amount of easily degraded organic matter in the inflows.
  • Inflow concentrations of both nitrogen and phosphorus are important to algal growth in the reservoir.
Impact of reservoir operations
  • Reservoir operations affect water quality.
  • Reservoir water levels and the length of time water stays in the reservoir affect TOC and dissolved oxygen concentrations.
  • Bottom withdrawals at Soldier Canyon Dam influence the duration of stratification and, consequently, the degree of oxygen depletion at the reservoir bottom.
Future Model Applications
The project has shown that a numerical model can successfully simulate water movement and water quality in Horsetooth Reservoir.  The model applies to many scenarios and can be used to help assess the impact of changes in operations, infrastructure and the watersheds on the reservoir’s water quality.
All models improve after application to new problems and as additional data and a better understanding of the modeled processes are obtained. Northern Water and stakeholders will continue to update and improve the model as they apply it to new scenarios.
Horsetooth Reservoir Water Quality Model Development
The project used the U.S. Army Corps of Engineers’ numerical model, CE-QUAL-W2, which is a two-dimensional (length and depth), laterally averaged, hydrodynamic and water quality model for lakes, reservoirs, rivers and estuaries.
This model has been applied to many reservoirs throughout the country and is well suited for long, narrow reservoirs such as Horsetooth.
It models water movement, or hydrodynamics, within the reservoir, thermal stratification, and water quality – including nutrient, algae, dissolved oxygen, organic matter, and sediment relationships.
Model inputs include reservoir bathymetry (geometry), meteorological data, daily inflows and outflows, inflow water temperature and water quality, initial water temperature and water quality conditions within the reservoir, and many hydraulic and water quality coefficients required by equations that govern the processes the model simulates. 
The model was calibrated using data from 2005 to 2009. Data from 2010 were used for validation.
The 2012 project included simulating a series of simple, preliminary “what if” scenarios designed to examine the impact on in-reservoir water quality due to changes in:
  • Inflow total organic carbon concentrations
  • Inflow nutrient (nitrogen and phosphorus species) concentrations
  • The length of time water is stored in the reservoir
Details about the model development and findings are documented in the Horsetooth Water Quality Study.