The concept of river health is an evaluation tool for river management. Its purpose is to establish a set of river ecosystem evaluation system and evaluate the changing trend of river ecological state in the long-term evolution process under the dual effects of external forces and human activities. . An ecosystem consists of two parts: living things and habitats. Life is the main body of the ecosystem, and habitat is the life support system. River is the carrier of biological life in water area, and it is also the medium of material flow and energy flow in aquatic ecosystem. In the evaluation system, it is necessary to pay attention to the process of river biological community, survival and evolution, and the coupling relationship between water biological community and river habitat. Hydrological conditions, water quality conditions and habitat quality are three factors that affect the health of river ecosystem. River depletion, cut-off and water pollution will lead to river ecosystem degradation and eutrophication. Habitat quality is an important stress factor for river health. Under the condition of constant water quantity and water quality, there is a linear relationship between biodiversity and spatial heterogeneity of habitats. The higher the spatial heterogeneity of habitats in a region, the more diverse the niches created, thus allowing more species to exist. On the other hand, if the abiotic environment becomes monotonous, the diversity of biological communities will inevitably decline, and the nature, density and proportion of biological communities will change, thus causing the degradation of ecosystems to some extent. River biological community is the main body of river ecosystem. Biological evaluation is to analyze the influence of changes in hydrological conditions, water quality conditions and habitat conditions on river biological communities. Possible changes include the change of species composition of aquatic community, the change of dominant population in habitat, species exhaustion, the death of the whole population, the change of biological behavior, the change of physiology and metabolism, the change of organization and the distortion of morphology.
The basic evaluation method is to compare the biological state of the reference river with the score. "Reference reach" generally selects the reach where water quality, river landform and biological community are basically undisturbed.
Because it is impossible to monitor and sample all the biome components in the river, the alternative is to choose a few symbolic species. There are several types of marker species:
(1) keystone species. Species occupy different positions in the ecosystem, and some rare large endemic species play an important role in maintaining biodiversity and ecosystem stability. Their disappearance or weakening may cause fundamental changes in the entire ecosystem.
(2) protect species. A species that is protected because of its rarity, culture or importance, or because its habitat is threatened.
(3) Umbrella species. Protecting this species and its habitat can protect a large number of other species that also depend on the same habitat.
(4) Flagship species. Refers to a symbolic creature generally accepted by the public. They often become attractive animals with their unique body shape, and their protection will protect the species born with them.
Specific symbolic species are usually selected from algae, large invertebrates and fish.
Generally speaking, coastal plants are not suitable as symbolic species because many plants are not sensitive to water pollution.