Uferzonen von Fliessgewässern in Kleineinzugsgebieten der Region Basel : geoökologische Prozesse, Nährstoff- und Wasserhaushalt, Bodendynamik, Kartierung, Funktionen und Zielbreitenermittlung

Riparian zones are important geoecosystems in fluvial topographies of the middle
latitudes, because they represent the transition zones between the river plane and the river.
The riparian zones are influenced by interactions of the fluvial system and the bordering
landscape. However, also structure of riparian zone on their part affect processes as well as
the water and nutrient balance of the neighbouring land and the streamlet. The dynamics of
these interactions lie in the focus of this study.
The aim of the project is to study geoecological processes as well as water and nutrient
balances in riparian areas. To do so, the preferential flow paths will be analyzed. Moreover,
the functions and influencing factors of riparian zones regarding water protection, such as
retention and removal are scrutinized.
The fieldwork for this study has been predominantly conducted in the Länenbach Valley, a
small catchment on the Swiss Jura Plateau. The Rüttebach Valley in Southern Black Forest
was chosen as complementary research area, since this area is characterized by different
geogenic and hydrological conditions. Furthermore, comparative studies will be conducted
for other rivers in the area of Basel.
To study the complex processes in riparian areas different methods are used: Eight research
plots are analyzed in high spatial resolution with pedological and hydrological techniques
concerning their nutrient dynamics. This is monitored on a weekly basis and by single
experiments. Consequently, a comparison of different riparian zones is possible.
The elements of connection of the investigated catchments are first a consistent
documentation of location properties and geoecological mapping of the areas. For the
analysis of the surveyed data, mostly specific or even especially designed methods are
used.
The results of the study are manifold and extensive. For instance, to standardize
geoecological mapping of riparian zones a GIS technique has been developed and tested.
This technique allows documenting the spatial distribution of the structures of riparian
zones more efficiently and less ambiguously.
Accompanying studies concerning the meteorological and fluvial dynamics provide an
insight into interactions of water input (mainly precipitation), the internal riparian
dynamics, and the fluvial output of water and soluble nutrients. Based on these findings,
the two investigated areas can be differentiated as follows: Vertical processes and ground
water enrichment are dominant in the Länenbach Valley. By contrast, lateral discharge in
the saturated zone characterizes processes in the middle reaches of Rüttebach Valley. In
both areas, subterranean interactions exist between infiltration-, river- and groundwater.
Soil analysis contain observations of vertical soil structures as well as small scaled lateral
variation of topsoil characteristics and are performed in several smaller projects. These
analyses clearly show that retention leads to enrichment, sedimentation, or intermediate
storage in the outer riparian zone. Thick ground vegetation generally promotes retention.
This, however, does not automatically lead to reduction respectively removal in the topsoil
of riparian zones if neighbouring riparian areas are intensively used for agriculture. Besides
self regulating processes auxiliary anthropogenic removal (for instance as part of mowing
and pruning) is a condition for general nutrient removal out of the riparian soil.
The investigation of surface processes reveals a clear picture: Small scaled wash-aquatic
processes are generally decreased from the riparian area to the riparian zone. As a
consequence, outer riparian zones are characterized by retention. In contrast, remobilization
happens particularly at steeper bank slopes. The river bank erosion further leads to
high discharge of soil sediments into the streamlet.
Several investigations – such as tracer experiments, measurements of infiltration, soil
moisture and soil water as well as groundwater survey – focus on subterranean flows of
water and nutrients. The quintessence of these investigations is that soil infiltration and
groundwater discharge are the dominant processes. If a location shows deep ground water
levels the preferential lateral water movement runs “beneath the riparian zone” towards the
streamlet. In that case, the riparian zone and its vegetation have a limited influence on the
water quality. In the same way anthropogenic drainage limits the retention and filter
function of riparian zones.
The general analyses reveal considerable quantitative differences in terms of flow paths.
These differences, however, do not correspond to the patterns of nutrient transport. In
contrast, pronounced variations exist for different parameters: Phosphorus is predominately
transported particle-bound on the surface, whereas nitrogen is mostly washed
out vertically and subsequently transported laterally in the saturated zone.
The dominant water flow paths are precipitation, evapotranspiration, vertical infiltration,
and lateral groundwater discharge. By contrast, solid material is generally transported
slowly and diffusely on the surface of riparian areas and then is predominantly discharged
by bank erosion.
The analysis gives strong evidence that the structures of riparian zones influence dynamics
of water and nutrients. However, the influencing factors are multifarious. In addition the
intensity of these factors varies over time and space.
Generally, the role of geoecological functions of riparian zones is often overestimated. For
example, in some cases high retention efficiency can be measured. The retention
efficiency, however, may only be temporarily, as in many cases the material is
subsequently remobilized laterally or washed out vertically.
The environmental policy in terms of the protection of riparian zones is complex. Conflicts
concerning land use in riparian areas are result of diverse interests and vague laws.
Therefore, it is necessary to optimize the width of riparian zones, their structures and their
care. The optimal site specific width of riparian zone can be calculated by an empirical
formula which is derived from the conclusion of this study. Since the actual width of
riparian zones can now be compared with the ideal width, policy makers and practitioners
have an easy-to-use instrument to assess the demand concerning the size of riparian zones.
Generally, the structures of riparian zones should be diverse, appropriately wide and along
the river uninterrupted in order to fulfil its actual geoecological function.