We consider the theoretical and methodological aspects and the experience of using methods for characterizing the ecological status (ES) of fluvial systems. The problems of development of multilevel and multi-criteria classifications of ES; stages and results of assessment research are discussed. Evaluation of ES can be performed on the basis of biocentrism in the study of fluvial systems or with an anthropocentric approach that takes into account the planning of fluvial systems use for various purposes with special attention to environment conservation and resource reproduction. Such an approach determines the axiological base and the choice of necessary and sufficient criteria and evaluation priorities. The assessment scales were developed in consideration of each component evaluation and indicator approach and can use incomplete, inaccurate, non-numerical assessments data. It was concluded that ES is an example of a system emergent property that combines its ability to produce organic substances in accordance with the natural history of fluvial systems, to maintain high quality of the habitat and biota, high biodiversity, resistance to changes in natural conditions and anthropogenic impacts; high velocity in the system of natural purification and low velocity of pollution and acidification; and maintenance of outlined system properties over a long time period. This
research is focused on the development of models for quantitative integral assessment of the state of fluvial systems, their systemic properties that are characterized by the emergence of new functional units (integral indicators of different groups, summarizing indicators) that reflect the system integrity, serve as the base for taxonomy, and allow comparison of the state of systems in space and time. Integral assessment assumes the implementation of assessment
with some uncertainty, taking into consideration different phases of the unification of multicriterial preliminary assessments with evaluation of their significance for overall assessment. This may take into account the significance of various indicators and their input to consolidated value. The development of integral indicators of ES is discussed in hypothetical examples of river drainage basin systems and on the base of assessment of ES of the river itself (Msta River)
and its drainage basin (landscape).