Substrate
Streambed substrate refers to the type of substance on the bottom of the stream. The cQHEI and QHEI measure the size of the substrates (e.g., from fine sediment various sizes of gravels and cobbles, up to boulders). Both cQHEI include measure of smothering by sands and other fines and from siltation.
Reference Condition
The condition that approximates natural, unimpacted, to best attainable conditions (biological, chemical, physical, etc.) for a waterbody. Reference condition is best determined by collecting measurements at a sufficient number of sites in a similar waterbody class or region under minimally or least disturbed conditions (by human activity), if they exist. Since undisturbed or minimally disturbed conditions may be difficult or impossible to find in some states, least disturbed conditions, combined with historical information, models or other methods may be used to approximate reference condition as long as the departure from natural or ideal is understood. Reference condition is used as a benchmark to establish numeric biocriteria.
Disturbance
Any activity of natural or human causes that alters the natural state of the environment and its attributes and which can occur at or across many spatial and temporal scales.