Stream Condition Index

Framework / Assessment
California Water Plan Update 2013: Pilot Statewide Assessment
Summary

This is a biological index, composed of indicators & metrics representing the condition of the benthic invertebrate communities living in streams and rivers. The presence and abundance of aquatic plants and animals can provide an indication of waterway and landscape disturbance, geomorphic conditions, appropriate water availability, and water quality. Comparing the measured presence (observed) of native species or groups to the expected presence of these species or groups is one way of measuring watershed and waterway conditions.

General Information
Sustainability Goal

Goal 5. Protect and enhance environmental conditions by improving watershed, floodplain, and aquatic condition and processes.

Domain

EH – Ecosystem Health = The condition of natural system, including terrestrial systems interacting with aquatic systems through runoff pathway WQ – Water Quality = The chemical and physical quality of water to meet ecosystem and drinking water standards and requirements.

What is it?

The presence and abundance of aquatic plants and animals can provide an indication of waterway and landscape disturbance, geomorphic conditions, appropriate water availability, and water quality. Comparing the measured presence (observed) of native species or groups to the expected presence of these species or groups is one way of measuring watershed and waterway conditions. The California Stream Condition Index is based on the comparison of the observed assemblage of benthic macroinvertebrate species to those expected from studying reference streams. It provides a scientifically robust way of assessing and describing conditions and tracking conditions over time or in response to regulatory or restoration actions. The State Water Resources Control Board has adopted the CSCI as a defensible and useful indicator of water quality and stream disturbance.

Benthic invertebrates are common, respond to environmental influences, and occur as diverse assemblages. By counting the number of individuals of different taxanomic and functional groups, assemblages can be described and inferences drawn about their aquatic environment. The Index uses comparisons of assemblages at "test" sites with "reference" (less-disturbed) sites, while taking into account natural variation. The Index has two components: 1) ratio of observed to expected taxanomic groups, and 2) proportion of the assemblage that falls into different functional groups that represent species diversity, ecosystem function, and sensitivity to stress. The Index is not normalized to a 0 to 1 or 100 scale, but instead compares Index values at test sites to values at comparable reference sites. The mean Index value of reference sites is 1.01. The 90th percentile value is 0.85. Streams with values >0.85 are considered to be "likely intact". The 99th percentile value is 0.72. Streams with values between 0.72 and 0.85 are considered to be "likely altered" and streams with values <0.72 are considered to be "very likely altered".

Why is it important?

The best way to assess the ability of a watershed to support living things is to look at those living things. Unlike chemical monitoring, for example, which provides information about water quality at the time of measurement, monitoring of living organisms (biomonitoring) can provide information about past and/or episodic pollution and the cumulative effects of a suite of watershed impacts. BMI represent ideal biomonitors for assessing the overall health of watersheds for a number of reasons:

1.    They are widespread
2.    They are easy to collect and identify
3.    They are relatively sedentary and long-lived, so reflect the longer-term effects of activities within their watershed
4.    Some species of BMI are highly sensitive to pollution

BMI-related metrics (e.g., taxa richness and diversity, specific taxa pollution sensitivities/tolerances, etc.) have been used by varied US agencies for many years as “bioindicators” of water quality, providing integrated information on toxic chemical concentrations, dissolved oxygen levels, nutrients, and habitat quality. Beyond their usefulness as bioindicators BMI are themselves an important part of aquatic food chains, especially for fish. Many BMI feed on algae and bacteria, which are on the lower end of the food chain. Some shred and eat leaves and other organic matter that enters the water. Because of their abundance and position as “middlemen” in the aquatic food chain, BMI play a critical role in the natural flow of energy and aquatic nutrients in streams, lakes and wetlands.

What can Influence or Stress Condition?

Many BMI are highly sensitive to changes in their aquatic environment and thus can act as continuous monitors of the condition of the water they live in. Human activities that interfere with or disrupt natural processes in a watershed can have significant impacts on the types and numbers of BMI that live there. Some BMI taxa require very good water quality, whereas others tolerate a wide range of environmental conditions. Although BMI can move about to some extent, drift downstream, and fly as adults, the aquatic forms generally cannot move quickly to avoid adverse conditions. Deteriorating water and/or habitat quality and pollutants can be expected to kill or at least stress less tolerant BMI taxa and encourage other more tolerant taxa to proliferate.

The CSCI is based upon comparison of an observed assemblage of BMI with an expected assemblage, based upon comparison with reference streams. With climate change, it is conceivable that conditions in streams previously thought of as “reference” will change and likely degrade. Therefore a decision will need to be made about whether or not the CSCI will remain an index of relative condition, where the benchmark is changing, or if the conditions that have been established at the creation of the index will serve as a benchmark into the future.

Target or Desired Condition

The CSCI was developed by the State Water Resources Control Board as a regulatory and informational tool to measure and protect water quality and stream processes. The desired condition is for streams to support native species and natural processes, including healthy trophic interactions and the full complement of expected species. The CSCI was developed using reference and test, or disturbed streams. The numeric desired target is the mean of the reference conditions (CSCI value = 1.01). The undesired condition is the absence of any expected native benthic macroinvertebrate species (CSCI value = 0).