This study presents the results of an extensive research program conducted between 1996 and 2014, which has led to a monographic synthesis over a territory with a particular significance in the whole geographical area of Oltenia Plain.
Generally, a basin includes, due to its dimensions, a wide variety of geographical, hydrological, physico-chemical, ecological structures. In this regard, taking into account the hierarchy of the river systems and the classification established by Strohler (quoted by UJVARY, 1959), water courses can be of the 1st, 2nd, 3rd, and 4th order, thus, defining the complexity of a hydrographical network within a hydrographic basin. Given how a hydrographical basin is defined, we discuss about a more or less particular case, due to its structural-functional features. It is the case of the Preajba Valley hydrographical basin. Located in a plain area, the system - placed in the catchment basin of the Jiu River - presents a hydrographic network of 30 km2. A wide variety of ecosystems is grouped within this space - small hills covered by pastures and meadows, farmlands and a complex river system: springs, streams, rivers, swamps, small reservoirs. The complex organization of the territory determines the structural and functional characteristics of the ecosystems.
Two human actions seem fundamental to the ecological evolution of freshwater ecosystems, but also for the evolution of human society: pollution (physical, chemical, biological) and hydrotechnical scheming of watercourses. Both caused and still cause changes in the structure and functioning of the natural ecosystems.
Hydrotechnical scheming - mainly the construction of dams, embankment and regulation of watercourses – aim at the utilization of water resources for multiple uses.
The construction of dams and reservoirs may have a dual effect on the river from the ecological point of view: a positive effect, by increasing the diversity of the types of ecosystems and of biodiversity, and a negative effect due to the imbalances of the biocoenotic structures and disruption of the physiology and behaviour of certain populations, especially fish (blocking reproduction or feeding migrations of fish).
The same situation occurs in case of the Preajba Valley River. On the one hand, we mention the enrichment of the natural landscape with new elements and the appearance of the lacustrine ecosystem with specific population structures, and on the other, the degradation of the rheophilic ecosystem located downstream the dam, due to the reduction of the water flow below the minimum necessary for the preservation and adequate functioning of the biocoenotic rheophilic structures.
We approached this issue as our research had to demonstrate to what extent the construction of dams and small reservoirs along a small tributary of the Jiu (the Preajba Valley River) induces changes in the evolution of the structures and functioning of plant and animal populations; thus, we focused on the study of the structure dynamics and of the production of gastropod populations.
In this context, during the research period, we aimed at the fallowing objectives:
• knowing the ecological features of the reservoirs;
• physical-geographical, morphological and hydrological structure of Preajba Valley reservoirs;
• chemical and biocoenotic structure of the reservoirs (water chemistry, planktonic and benthonic communities);
• emphasizing the present state of knowledge of gastropod biology (structure of freshwater gastropods, their distribution related to the zonality of the hydrographical network of Romania);
• establishing the structure of the gastropod populations from Preajba Valley reservoirs (taxonomic structure, spatial distribution, structure on age categories, structural biocoenotic indices and numerical variations of gastropod populations during different seasonal stages);
• evaluation of biomass variations as a reference factor for gastropod production.
The ecological peculiarity of the Preajba Valley hydrographic basin is that, within a limited geographical space, there is grouped a wide variety of ecosystems (springs, streams, rivers, swamps, small reservoirs), which confers it a unique character within Oltenia Plain and even Romania.