Although it flows through the centre of the German capital, the Spree is a river that has so far attracted little interest from historians. Yet this almost 400 kilometre-long tributary of the Havel in eastern Germany was and is not only central to ship transport and the supply of drinking water and wastewater disposal for the city of Berlin. It is also at the centre of the far-reaching changes to the landscape around the open-cast lignite mine in Lower Lusatia, of which it is the most important water drain. At the same time, it irrigates the tourist attraction, the Spreewald near Berlin, and its lakes are an important recreational area for the Berlin population. During the period of German-German division, the Spree played an important role both as a barrier and as an escape route as a river that did not respect political borders. Like many other rivers, the Spree was straightened and dammed, equipped with sluices and tunnelled under, its banks fortified with walls and built on, its natural course altered, especially in urban areas, and its water polluted with filth and chemicals of all kinds.
How could one write an environmental history of the Spree that takes all these aspects into account and that at the same time makes the many facets of human perception of this 'lifeline' tangible? One that consults not only traditional maps, but also serial sources of all kinds that deal with the state of the river? That takes into account very personal experiences with the river and also honours literary sources? Which represents the various forces acting on the river - be they natural or man-made? So much for the epistemological questions. But such a complex and colourful topic also deserves appropriate visualisation, not least in the form of maps. How can the spatial aspects of the river's history be visualised in a compelling way? Are there cartographic methods that make it possible to depict not only the multidimensionality of the history of a river, but also the diversity of experiences of its neighbours and users over the course of history?
Some more recent approaches to the integration of different levels in map visualisation are the concepts of 'deep', 'thick' or 'fuzzy mapping' developed mostly in the Anglo-American world. They transcend the classic two-dimensional maps and depict historical processes in multi-dimensional views, linking a wide variety of layers, some of which are also intended to depict virtual realities. They are not limited to the representation of present and past realities, but also include unrealised spatial arrangements or future plans of a space or place.
While 'deep' and 'thick mapping' have already been tested on a few historical regions and locations, fuzzy mapping, which originated in the field of IT and neurology, has not yet been used to answer historical questions. The method seeks to capture and depict causal knowledge and represent cognitive landscapes in the manner of neural networks and has been used to model decision-making processes in social and political systems. For the representation of structured knowledge and the modelling of complex systems, the method is attracting increasing research interest in various scientific disciplines. With this task and the tools developed so far, it should also be possible to visualise environmental historical processes in the form of maps, perhaps even better than is possible with existing methods.
Using the history of the Spree in the 19th and 20th centuries as an example, a variety of different source and data resources (such as water levels, leisure traffic, pollution, fishing results, etc.) or even quotations from serial sources such as postcards, inspection reports, etc. will be interlinked in the sense of 'deep' or 'thick mapping' in order to create a multidimensional picture of the environmental history of the river, its space and its inhabitants. Finally, fuzzy mapping will be used to visualise development and decision-making processes with their causalities and drivers and unrealised alternatives. The paper presents initial research results and raises further methodological questions and problems.