Vegetation growing in the water along watercourses has been the subject of several studies since it was recognized that it could have a significant impact on the water flow. It may increase resistance to flow and cause higher water levels. Also, it has an effect on the velocity profiles. Previous investigations on the flow of water through emergent vegetation have shown different results. The purpose of this paper is to investigate, and determine how aquatic vegetation influences flow resistance, water depth and discharge in the Chotárny channel at the Žitný Ostrov area. This area is part of the Danube Lowland (south-west of Slovakia). The channel network at the Žitný Ostrov region was built up for drainage and also to provide irrigation water. The Chotárny channel is one of three main channels of this network. Measurements performed during six years at this channel were used for an evaluation of vegetation impact on flow conditions. The roughness coefficient was used as one way of quantifying this impact. The results show variation of this parameter during the growing season. Vegetation causes resistance to flow; it reduces flow velocities, discharge and increases water depth.

The Water Framework Directive requires as an obligatory goal to achieve and to keep "good water quality" status within the defined period (for Slovakia-up to the year 2015). For surface waters, the main criterion is the ecological and chemical status of the water. Mathematical and numerical modelling allows evaluating various situations of contaminants spreading in rivers (from everyday wastewater disposal through fatal accidents and discharges of the toxic substances) without immediate destructive impact to the environment. Determination of longitudinal and transverse dispersion coefficient values, as the main hydrodynamic characteristics of the dispersion, has the highest extent of uncertainty for hydrodynamic models simulating pollutant transport in streams. This paper deals with the determination of dispersion coefficients based on field tracer experiments performed in a small modified stream (basic hydrodynamic parameters during the experiments were: discharge Q = 0.138-0.553 m 3 .s −1 , depth h=0.29-0.48 m, width B=5.2-5.9 m). During the experiments, various conditions and situations were taken into account, e.g., continuous and instantaneous pollution source, as well as various positions of pollution source along the river width, among others. Field measurements were evaluated using three different methods for dispersion coefficient determination: based on statistical evaluation, based on analytical solutions of advection-dispersion equation, and using numerical models. The dimensionless dispersion coefficients values were determined, which can be used for numerical simulation of pollutant transport in similar types of streams.

Aim of the paper was to evaluate the changes of bed silt state of the Rye Island channel network in the period from 1993 to 2004. The Rye Island is part of the Danube Lowland, it is area between the two branches of the Danube river-the Danube and the Small Danube. During previous centuries (up to the 19th century), the Danube river branched into multiple streams and frequently changed course within its own alluvial sediments. At present, the process of river migration has stopped or is being controlled. Measurements of bed silt thickness in Rye Island channel network were done in 1993 at the channels: Aszód, Gabčíkovo-Topoľníky, Aszód-Čergov, Čergov-Komárno, Čalovo-Holiare and Holiare-Kosihy. Then in 2004 were done measurements at selected profiles of Aszód, Gabčíkovo-Topoľníky and Komárňanský channel for checking of the silting up variability. According to results of measurements at the three selected channels we can say that the channel silting up has not been changed considerably in monitored period. The observed facts could be useful for channels maintenance, as a base for expectation of silting up process in the channels. Also the knowledge about silt thickness in the channels is important from the view of interaction between channel network and groundwater, especially when this information would be supplemented by characteristics of silt permeability. So the other aim of field measurements was determination of hydraulic conductivity. Their values were calculated by empirical formulas. These characteristics will be used for simulation of interaction between channel network and groundwater at the Rye Island area in the oncoming period.