Water Quality

Water quality refers to the suitability of water for sustaining ecological processes and supporting designated human uses, based on its physical, chemical, and biological characteristics. It is a cornerstone of river health and a key component of ecological integrity, alongside physical and biological integrity. Assessing water quality involves measuring a range of parameters that indicate the water’s condition and potential stressors.

Key parameter groups for water quality typically include:

To protect human health and aquatic ecosystems, regulatory agencies (typically states, under the framework of the federal Clean Water Act in the U.S.) establish water quality standards. These standards define the water quality goals for a specific water body by designating its beneficial uses (e.g., coldwater aquatic life support, primary contact recreation, public water supply, agriculture) and setting criteria (numeric concentration limits or narrative descriptions) necessary to protect those uses. Monitoring data is compared against these standards to determine if a water body is meeting its designated uses or if it is “impaired”.

In Colorado common uses of water include support for coldwater fisheries in mountain streams, warmwater fisheries in lower reaches, agricultural irrigation, municipal and domestic water supply, and various forms of recreation. Water quality concerns vary across Colorado’s diverse landscapes. Salinity and total dissolved solids (TDS) can be naturally elevated in arid basins or increased by irrigation return flows. Legacy and ongoing mining activities contribute dissolved metals in many mountainous areas. Nutrient enrichment from agriculture and wastewater impacts streams in populated valleys. Water temperature is a critical issue, particularly for coldwater fish, and is threatened by factors like reduced flow, loss of riparian shade, and climate warming. Fecal contamination from livestock or human sources is also a concern in some areas.

Component: Dissolved Metals

Dissolved metals refer to metallic elements present in ionic form within the water column. Common metals of concern in river ecosystems include copper (Cu), lead (Pb), zinc (Zn), cadmium (Cd), mercury (Hg), arsenic (As), selenium (Se), iron (Fe), and manganese (Mn). Their concentrations can fluctuate depending on source inputs, streamflow, water chemistry, and interactions with sediments.

Natural sources contribute background levels of dissolved metals through the chemical weathering of rocks and soils. The specific geological setting of a watershed plays a major role. Regions with naturally mineralized rock formations can have inherently higher background concentrations of certain metals. Geothermal activity can be another natural source of metals like arsenic and mercury.

Anthropogenic activities are often the primary cause of elevated and harmful metal concentrations in rivers. Mining operations are a particularly significant source, especially in historically mined regions. Acid mine drainage (AMD), formed when water reacts with sulfide minerals exposed during mining, can release high concentrations of toxic metals and acidity into streams, devastating aquatic life. Leachate from mine tailings piles and waste rock can also contribute metals, even long after mining has ceased. Other human sources include industrial wastewater discharges, atmospheric deposition from smelting or fossil fuel combustion, urban runoff carrying metals from vehicles and infrastructure, and some agricultural inputs. Energy extraction activities, such as uranium mining or the disposal of produced water from coalbed methane extraction, can also introduce metals and radioactive materials into water bodies.

The primary concern regarding dissolved metals is their toxicity to aquatic organisms. Many metals, even at trace concentrations, can impair physiological functions, reduce growth rates, inhibit reproduction, cause developmental abnormalities, and lead to mortality in fish, invertebrates, and algae. Sensitivity varies among species and life stages. Metals can also bioaccumulate, increasing in concentration within organisms over time, and biomagnify, becoming more concentrated at successively higher trophic levels in the food web. This poses risks not only to aquatic predators but also to terrestrial wildlife and humans who consume contaminated fish. Elevated metal concentrations can render water unsuitable for drinking or irrigation.

Component: Nutrients (Nitrogen and Phosphorus)

Nitrogen (N) and phosphorus (P) are essential nutrients required for the growth of primary producers—algae and aquatic plants (macrophytes), which form the base of most aquatic food webs. These nutrients occur naturally in various dissolved inorganic forms (e.g., nitrate [N.-], ammonium [N.+], phosphate [P.-]) and organic forms, with background concentrations influenced by watershed geology, soils, vegetation, and atmospheric inputs.

While essential in limited amounts, excessive inputs of N and P from human activities lead to cultural eutrophication, the over-enrichment of water bodies. Major anthropogenic sources include:

  • Agriculture: This is often the dominant source in many developed watersheds. Runoff and leaching from agricultural lands carry excess nutrients from chemical fertilizers and animal manure. Phosphorus tends to bind strongly to soil particles and is primarily transported via erosion, while nitrogen is more water-soluble, especially nitrate, and readily moves through surface runoff and groundwater leaching. Ammonia can also be lost to the atmosphere from manure and fertilizers and redeposited into waterways.

  • Wastewater: Effluent from municipal sewage treatment plants represents a significant point source of both N and P, although advanced treatment can reduce these loads. Failing septic systems in unsewered areas contribute nutrients via groundwater.

  • Urban Runoff: Stormwater runoff from urban and suburban landscapes carries nutrients from lawn fertilizers, pet waste, detergents, and atmospheric deposition on impervious surfaces.

  • Industrial Discharges: Certain industries can discharge nutrient-rich wastewater.

  • Atmospheric Deposition: Nitrogen oxides released from the combustion of fossil fuels can be deposited into watersheds, contributing to nitrogen loading.

The ecological consequences of eutrophication can be severe. Increased nutrient availability stimulates excessive growth of phytoplankton - microscopic algae suspended in water - and/or periphyton - algae attached to surfaces - and macrophytes. The decomposition of large amounts of dead algal and plant biomass by bacteria consumes large quantities of dissolved oxygen (DO) from the water column, particularly near the sediment surface. This can lead to hypoxia (low DO levels) or anoxia (complete absence of DO), creating “dead zones” where fish, shellfish, and most benthic macroinvertebrates cannot survive. Dense algal blooms also increase turbidity, shading out beneficial submerged aquatic vegetation. Certain types of algae, particularly cyanobacteria (blue-green algae), can produce potent toxins during blooms (Harmful Algal Blooms or HABs), which are harmful or lethal to fish, wildlife, domestic animals, and humans through direct contact or consumption of contaminated water or shellfish. Eutrophication fundamentally alters aquatic community structure and food web dynamics, often favoring tolerant species over sensitive ones. Nutrient pollution is a widespread issue, impacting rivers and lakes in agricultural and urbanized settings across Colorado.

Component: Physical Parameters

Physical water quality parameters describe the physical state of the water and strongly influence biological processes and habitat suitability.

Water Temperature:

  • Characteristics: A measure of the heat energy in the water. Natural river temperatures fluctuate daily and seasonally, driven by solar radiation, heat exchange with the atmosphere, heat exchange with the streambed, and the temperature of surface water and/or groundwater inflows. Riparian vegetation plays a crucial role by providing shade, which reduces direct solar heating. Channel structure and flow volume also affect how quickly water heats or cools. Groundwater inputs tend to buffer temperatures, providing cooler water in summer and warmer water in winter relative to surface influences.

  • Influence/Impacts: Temperature is a critical “master variable” for aquatic life, controlling metabolic rates, growth, behavior, reproductive success, and survival limits for aquatic organisms like fish and invertebrates. Each species has an optimal temperature range and upper/lower lethal limits. Temperature dictates the suitability of a river reach for coldwater species versus coolwater or warmwater species. Temperature also directly affects the solubility of dissolved oxygen as colder water holds more DO and influences the rates of chemical reactions and the toxicity of certain pollutants.

  • Human Impacts: Human activities frequently alter natural thermal regimes. Thermal pollution can occur through the discharge of heated water from industrial processes or power plant cooling systems. Impoundments (dams and reservoirs) have complex effects: releases from the warm surface layer of stratified reservoirs can increase downstream temperatures in summer, while releases from the cold bottom layer can cause unseasonably cold temperatures downstream (“cold water pollution”), disrupting life cycles of native fish adapted to natural seasonal warming. Dams also tend to reduce natural diurnal and seasonal temperature fluctuations. Removal of riparian vegetation eliminates shade, increasing solar radiation reaching the water surface and leading to higher summer temperatures, particularly in smaller streams. Reduced streamflow due to water withdrawals or diversions decreases the water volume, allowing it to heat up more quickly. Channel widening resulting from altered flow or sediment regimes increases the surface area exposed to solar radiation. Climate change is causing a general warming trend in rivers globally, posing a significant threat to coldwater species.

Dissolved Oxygen (DO):

  • Characteristics: The concentration of molecular oxygen dissolved in water, typically measured in milligrams per liter (mg/L) or percent saturation. DO levels are governed by the balance between processes that add oxygen (reaeration from the atmosphere, photosynthesis by aquatic plants/algae) and processes that consume oxygen (respiration by all aquatic organisms, decomposition of organic matter by bacteria). Atmospheric reaeration is enhanced by turbulence in riffles. DO saturation is inversely related to temperature and salinity since colder, fresher water holds more DO.

  • Influence/Impacts: DO is essential for the survival of most aquatic animals, including fish and macroinvertebrates, which require it for respiration. Different species have different minimum DO requirements; coldwater fish generally require higher DO levels than warmwater fish. Low DO levels or hypoxia cause physiological stress, reduced growth, avoidance behavior, and, if severe or prolonged, mortality. Anoxia or zero DO is lethal to most aerobic organisms. DO levels are a key indicator of overall water quality and ecosystem health.

  • Human Impacts: Activities that increase organic matter loading, such as sewage discharge, agricultural runoff with manure or plant residues, stimulate bacterial decomposition, which consumes DO - measured as Biological Oxygen Demand, BOD. Nutrient enrichment leading to eutrophication causes large swings in DO, with high levels during daytime photosynthesis but potentially severe depletion at night and during bloom decomposition. Increased water temperatures reduce DO saturation capacity and increase organismal metabolic rates. Reduced flow or impoundments decrease turbulence and reaeration rates. Discharges from the anoxic bottom waters of stratified reservoirs can release water with very low DO downstream.

Turbidity and Suspended Sediment:

  • Characteristics: Turbidity is a measure of water clarity or “cloudiness,” caused by suspended particles like silt, clay, fine organic matter, and microorganisms like algae. Total Suspended Solids (TSS) is a direct measure of the mass of particulate matter suspended in a volume of water. These parameters often correlate but measure slightly different things. Levels naturally fluctuate with flow; turbidity and TSS are typically low during baseflow and increase significantly during storm events or snowmelt due to erosion and sediment resuspension.

  • Influence/Impacts: High turbidity reduces light penetration, limiting photosynthesis by submerged aquatic vegetation and algae. Suspended sediments can physically harm aquatic organisms by abrading or clogging gills in fish, interfering with filter-feeding mechanisms, and reducing visibility for sight-feeding predators. As suspended sediments settle out, they can smother bottom habitats, filling interstitial spaces in gravels crucial for macroinvertebrates and fish egg incubation, and burying benthic organisms. Sediment particles can also transport adsorbed pollutants like metals and pesticides. While excessive sediment is harmful, extremely low turbidity might increase predation risk for some species.

  • Human Impacts: Activities that increase erosion are the primary cause of elevated turbidity and TSS. These include land disturbances like construction, agriculture, logging, mining, road building, and wildfire, which increase runoff and sediment delivery to streams. Channel modifications like dredging or bank destabilization can directly resuspend sediments. Urban runoff carries fine sediments from impervious surfaces. Wastewater discharges can also contribute suspended solids. Conversely, dams trap sediment, leading to artificially clear water releases downstream, which can alter downstream channel dynamics and ecology.

Interplay with Other Components

The interconnectedness of water quality parameters is evident. Temperature directly affects DO solubility and biological rates. Nutrient levels drive algal growth, which influences turbidity and DO via photosynthesis and decomposition. Sediment levels impact turbidity, temperature, and habitat suitability, which in turn affects biological communities and their influence on DO and nutrients. Furthermore, the flow regime exerts strong control over water quality. High flows can dilute pollutants but also increase erosion and transport of sediments and associated contaminants. Low flows reduce dilution capacity, concentrating pollutants, and can exacerbate high temperatures and low DO conditions. Understanding water quality requires considering the interplay between chemical constituents, physical conditions, biological activity, and the prevailing flow regime. Alterations to flow often trigger secondary changes in water quality parameters, such as temperature and sediment transport.

Colorado Water Conservation Board

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