Groundwater table levels in a river valley depend, among other factors, on meteorological and hydrogeological conditions, land use and water levels in watercourses. The primary role of a watercourse is to collect surface and groundwater, and it becomes an infiltrating watercourse at high water levels. Changes in groundwater levels and the range of these changes depend chiefly on the shape, height and duration of the flood wave in the river channel. The assessment of flood wave impact on groundwater was based on long-term measurements of groundwater levels in the Odra valley and observations of water levels in the river channel. Simulations were performed with the use of in-house software FIZ (Filtracja i Zanieczyszczenia; Filtration and Contamination), designed for modelling unsteady water flows within a fully saturated zone. A two-dimensional model with two spatial variables was employed. The process of groundwater flow through a porous medium, non-homogeneous in terms of water permeability, was described with Boussinesq equation. The equation was solved with the use of finite element method. The model was applied to assess groundwater level fluctuations in the Odra valley in the context of actual flood waves on the river. Variations in groundwater table in the valley were analysed in relation to selected actual flood water levels in the Odra in 2001-2003 and 2010. The period from 2001 to 2003 was used to verify the model. A satisfactory agreement between the calculated and the measured values was obtained. Based on simulation calculations, it was proved that flood waves observed in 2010 caused a rise in groundwater table levels in a belt of approximately 1000 metres from the watercourses. It was calculated that at the end of hydrological year 2009/2010, the highest growths, of up to 0.80 m, were observed on piezometers located close to the Odra river channel. The passage of several flood waves on the Odra caused an increase of subsurface retention by 3.0% compared to the initial state.

The paper contains analysis of ice phenomena occurrence in the light of recent observations conducted on middle Odra river from Krapkowice (ca. km 125+000) to Krosno Odrzańskie (ca. km 530+000). In detail the research embraced time of occurrence and intensity of individual ice phenomena in years 2006–2017 together with reference and comparison of data to previous data from period 1957–2005. In characteristic cross-sections there is a reference to occurrence of all observed forms of ice phenomena (frazil ice, border ice, ice cover, floe and ice-jam). In the study a reference data of Regional Water Management Authority in Wrocław were used together with results of previous research conducted at the Wrocław University of Environmental and Life Sciences. The results legitimized, that the occurrence of ice phenomena on middle Odra river has time of its appearance, duration and intensity and is not stable in every year. Occurrence of ice phenomena on middle Odra river varies in time, being observed as manifold forms of icing – most frequently border ice and frazil ice. During the last 12 years greater than the average number of days with ice phenomena occurrence can be observed, but these phenomena have definitely “milder” character (border ice).