Until the advent of modern power sources such as steam and combustion engines, the small rivers in Middle Europe played a central part in the economy as suppliers of mechanical power, water and means of transport. As a result, virtually all small rivers host a variety of remains of previous industrial activities such as weirs, mill races, ponds etc. which were built in the era ranging from the Middle Ages to the beginning of the 20th Century. Today, these remains are often not even recognized as such any more by hydraulic engineers, planners and the general public. The canalization of many rivers during the 1960's and 70's destroyed many remnants of the pre-steam age industries along the small rivers. As a result of the increasing ecological awareness, the concept of re-naturalization was developed in the 1980's. The current 'ideal' aim of re-naturalization, of re-creating a 'natural' river, does not take the historical context of our rivers into account, so that re-naturalization as well as the 'hard' engineering solutions can both result in the irretrievable loss of the cultural heritage. A current research project conducted by the authors tries to raise awareness of the historical context and to develop concepts of how to integrate the remains of the industrial past into the changing demands on our small rivers whilst contributing to the overall quality of the environment.

Before the advent of steam power, the small rivers played a vital part in the economy of the Central European countries as well as in North-America. Rivers supplied mechanical power for industry, irrigation water for agriculture, drinking water for the population, water for defense systems and they were used as transport ways for bulk goods. In order for the rivers to fulfill all these duties, and to ensure that the course of the river remains stable, man made very significant changes to the small rivers ever since the Middle Ages. These changes include the construction of weirs, fortification of banks, change of river course, addition of diversion or transport canals, creation of lakes for water storage, and many other items. With the development of first steam power and then electricity and the internal combustion engine, the use of the small rivers by man declined until it reached a point where they are hardly used at all.

Following the decline of usage, and the increasing demand for rural and urban space, many small rivers were forced into straight beds, concrete channels or even underground, changing the river from an ecosystem into a lifeless drain. This happened in particular during the 1960's and 70's. With the development of an increasing consciousness for our environment, other ecologically acceptable solutions for the small rivers were sought for. The solutions principally include the re-naturalization or return of the river to its natural state, using traditional construction methods and natural materials. The river would therefore be allowed to develop its own dynamics and ecosystem. Re-naturalization is today seen by many scientists, engineers and landscape architects and subsequently the public as the most desirable development strategy for rivers.

Many hydraulic engineers are not fully aware of the extent of the former use of the small rivers, or of the purposes they were used for. Quite often the redevelopment of a river - be it a 'hard' engineering solution or a 'soft' re-naturalization - therefore destroys the remnants of forgotten industrial activities not by intention but by lack of knowledge and awareness. These remnants however constitute a significant part of our cultural heritage and indeed of the development of engineering, which are subsequently and irretrievably lost.

When engineers in Germany talk about 'Kulturlandschaft' (cultivated landscape), they invariably mean the landscape shaped by man's current activity like agriculture, construction, infrastructure and so on. Archaeologists however have a rather different definition of this term. For them, 'Kulturlandschaft' means the landscape which was shaped by man for hundreds and thousands of years, and which -for today's inhabitants often invisibly - contains significant parts of our cultural heritage. Very often the remains of this field of agricultural and industrial activities are overlooked simply for lack of knowledge about our pre-steam era industrial past both on the side of engineers and the general public. In addition, the question arises as to how far an ecosystem which developed within a man-made environment for up to eight centuries can not be considered as natural. Modern textbooks on river hydraulics and river re-naturalization often completely omit the historical context, e.g. Patt et al. (1998). Only very recently has the term 'Kulturlandschaft' been introduced into the engineering field, Hintermeier (2003). Today, the extent to which the small rivers were formerly used, and the importance this usage had to society, is hardly known even to specialists working in this field such as hydraulic engineers. Therefore the remains of the former usage are often not even recognized as such.

A small comparison may illustrate the importance of rivers as power source and means of transport in times before steam or combustion engines existed: a typical water wheel of 10 kW provided more, cheaper and more reliable power than 30 to 40 horses or one hundred men; a horse drawing a boat on a canal could shift 40 - 50 times the weight it could move on a road. Water courses were therefore of prime importance. In the following, some of the principle uses of small rivers and their features will be presented in order to illustrate the way and the extent to which rivers were changed.

The water wheel is one of man's oldest hydraulic machines; water wheels were already described by the Roman architect Vitruvius (in ‚De Re Libri Architecturi XII'). A water wheel installation required a head difference in the river, which was usually provided for by a weir. The wheel installation itself was located at a mill race (today it would be called a 'diversion canal'). Apart from wind mills, water wheels were the only mechanical prime movers before the advent of steam engines and drove flour mills, textile and mechanical machinery, powder and mineral mills, water pumps for drinking water supplies etc, Reynolds (1983). It is estimated that in 1850 there were 25 - 30,000 water wheels in operation in England, 6,400 in Ireland and around 40,000 in Germany. Even as late as 1925, there were still 33,500 water wheels in operation in Germany with individual capacities of 1 - 75 kW and an overall capacity of 560 MW. This technology eventually disappeared in the 1950's and is today already virtually forgotten. Figure 1 shows a typical mill installation (River Aschaff / Bavaria 1889) with a weir, inflow detail, mill race, mill building with water wheel and outflow. Water wheels are today regarded as an outdated technology belonging to the romantic ages. Some recent research has however shown that water wheels had been developed into very efficient energy converters. Figure 2 shows some typical water wheels and the measured efficiency curves which indicate the surprisingly high efficiencies of 75 - 85%. More technical information about water wheels can be found in Müller & Kauppert (2002, 2003)
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Traditionally, city fortifications are envisaged by most people as walls with towers, possibly with a water filled ditch in front of the wall. This type of fortification was in use until, at the end of the 15th Century, modern cannons were introduced which simply shot stone walls apart in a very short time. As a result, fortification design changed dramatically during the 16th century and a system of earthen ramparts, bastions, outworks and major hydraulic elements, some of which are still visible today, evolved. The City of Braunschweig in Northern Germany is one of the few German cities where even the river course was changed in order to create a wet ditch for the artillery fortification, and where this altered course is still in existence. Today, not even the name of the river branches (‚Umflutgraben', flood diversion canal) recalls their original purpose. Figure 3 shows two maps of the City from 1650 and from today. In Figure 3a the original course of the River Oker through the city centre is still visible. The river channel running through the city center was maintained, probably for transport purposes, and weirs and mill races were built in
order to power mills inside of the city in case of a siege. These canals are also visible, as is the very narrow wet ditch of the mediaeval fortifications (thick dark line).

In Figure 3b, most of these items have disappeared and only small sections of the medieval ditch and the wet trench of the 17th Century fortification still exist.

The use of rivers for transport did not only mean navigation, but also e.g. the floating of timber. Often, special float canals were built alongside the river to provide sufficient depth and width of water for floats; as can be seen in Figure 4 for the River Aschaff. Apart from transport, in particular very small water courses were often used for irrigation purposes. The run of such small water courses was therefore frequently changed from its typical location at the lowest point of a valley to a zig-zag course running from the low point to the edge of the valley further downstream. This new and higher course enabled parts of the valley to be irrigated from above, and reduced the river's gradient and dynamics.

In addition, ponds were created for purposes such as storage of logs, provision of water to float logs downstream, fish farming, provision of additional water for mills during daytime etc. In many instances rivers were made navigable by building weirs and possibly side canals; this aspect is also very interesting both from the point of view of cultural heritage and the re-use today for leisure boating. This topic is however beyond the scope of this article.

One characteristic of historic (i.e. before the 1900's) alterations of river courses was that usually only those small sections were changed where a change was absolutely required, such as the addition of a mill race. The development of hydraulics as an engineering discipline led to a situation where small rivers were perceived by engineers purely as channels for rain- and groundwater. During the 1960's and 70's many small rivers were subsequently straightened over long stretches, often using concrete linings, in order to minimize the land required by the river (e.g. to maximize farmland or inner city land usage) or to protect roads and railway lines. This straightening generated a number of very undesirable side effects, such as a deepening of the river bed and the unfavorable superposition of flood waves caused by their accelerated propagation. The rivers running in concrete channels could not support eco-systems or even trees any more, and degenerated into lifeless drains. Following the increasing ecological awareness of the public, engineers tried to develop a concept for an ecologically and hydraulically acceptable alteration of water courses.

The re-naturalization of rivers is a concept which is today regarded by biologists, ecologists, landscape planers, by the public and by many engineers as the most desirable development. The main goal of re-naturalization is to bring the river as close to its original or 'natural' state as possible by removing artificial obstacles such as weirs, by changing of canalized sections, by provision of fish passes, of space for flood plains etc., e.g. Patt et al. (1998). In addition, re-naturalization tries to create a water-land interface to create a living space for aquatic and an access area for land based animals and to increase flood retention areas, although the latter aim is difficult to achieve due to the high demands for space. For these purposes, 'soft' construction techniques using natural material like willow branches, wood and rock were developed.

"One of the primary aims of the national and international effort for the protection of rivers is the creation or re-creation of a free passage; ...therefore weirs and barrages can be regarded as a significant anthropogen disturbance of the ecosystem", Schrenk (2003). The German recommendations for the management of small rivers which are currently being developed give a 'potentially natural state of the water course' as the development aim. The 'potentially natural state' is defined as an ideal conceptual state of future development of the water course which is undisturbed by man. Although this ideal is defined, current practice is to bring water courses 'close to a natural state' rather than creating a truly natural state. The latter concept is very difficult to achieve due to a variety of reasons such as land interests, the proximity of infrastructure installations such as roads, railways, gas, water, sewage and electricity lines etc. which must not be interfered with, recreational use and other reasons.

A review of the interference of man with the rivers since the 1960's however shows that 'hard' engineering solutions and current re-naturalization concepts have one peculiar aspect in common: the historical context is completely omitted, and the implementation of either concept implies the destruction of all historical artefacts along the river.

The River Elsava (NW Bavaria) is a typical small European river with a length of approximately 15 km, a drained area of 142 km² and an average flow rate of 1.23 m³/s. This river has been used by man since the middle ages for power production and irrigation and a large number of man-made features still exist along the river. A water mill at the old monastery of Himmelthal (Elsava) was first mentioned in 1232, water rights for the extraction of water for irrigation purposes were recorded in 1435 (Elsava). It can therefore safely be assumed that weirs and side channels have existed for 600 - 750 years. Figure 5 shows a stretch of around 3000m of the river Elsava in North-West Bavaria (flowing into the River Main). On this figure alone, 3 mill races and two irrigation canals are visible. In all, the Elsava once had 12 mills of which one, a saw mill (Kreuzmühle), is still in operation. Both irrigation channels are also still in use, one of them to supply cooling water to a chemical factory. The Elsava today constitutes a small river reasonably unaffected by dramatic changes, which has preserved the main features it has had for many centuries. The Archeological Project Spessart (see internet references) is trying to raise the consciousness within the population about this heritage of the age of small hydropower and early industry in order to prevent the disappearance of mill races by accident or re-naturalization and possibly to discover new usages for the existing hydraulic systems.

This river, a tributary of the River Main (NW Bavaria) with length of 8.4 km, a drained area of 144 km² and an average flow rate of 1.34 m³/s once powered nine water mills and was also used as a way to transport logs by floating them down river. For these purposes, weirs were built and artificial lakes created to store wood under water or to provide a mass of water to float the stored wood downstream over shallows and obstacles. When a weir and power station was constructed in the river Main in 1920, the estuary of the Aschaff had to be relocated downstream of the weir in order to maintain the gradient and the water rights connected with the Aschaff, see Figure 6 (Aschaff 1917 - 1970)
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Figure 6. Aerial photography of the confluence of the River Aschaff with the River Main, with alterations from 1917 to 2001
The old estuary was subsequently replaced ("Mündung bis 1920") and the lower course of the Aschaff was straightened in 1970, whereby all the old mills, weirs, diversion canals, mill ponds and mill races were destroyed and disappeared, see Figure 7a. Figure 7b shows the canal-cross section then built. The Archeological Project Spessart has documented the changes of the river and established a 'Heritage Walking Course' with display boards to familiarize people with the ongoing changes, the previous importance of the river and thus the reasons for today's situation (including the loss of many aspects of the heritage).

>From 2001 to 2002, an upstream section of the River Aschaff was re-naturalized, giving it a meandering plan view with ragged banks and shallow water zones which provide an suitable environment for aquatic animals. The course of the River had to be fixed in order to avoid interference with a newly built motorway and a variety of other infrastructure installations. Figure 8 shows today's course of the river with plan views of the two mills which existed before the re-naturalization with mill races, side channels and weirs. Again it can be seen that all remnants of the previous use of the river have disappeared in favor of a naturally-looking river. The River Aschaff demonstrates visibly the effect of the alterations of the last 40 years which were implemented following the prevailing river development philosophy but disregarding the historical context.

Figure 8. Re-naturalized section of the Aschaff with former mills (WWA Aschaffenburg, 1889)

The Wallensteingraben in North-Eastern Germany today looks like a natural water course, draining Lake Schwerin towards Wismar and the Baltic Sea. In reality, it constitutes the remains of a small canal built at the end of the 16th century to further Wismar's salt trade. The canal was finished in 1580, with a total length of 15.5 km and a height difference of 37.60 m which was overcome with 12 locks. Due to political developments, mainly caused by the 30 Years War (1618 - 1638) and the fact that Wismar became Swedish, depriving the city of its hinterland, the trade and with it the requirement for the canal practically disappeared. Subsequently the canal fell into disrepair. It still acted as a draianage for the lake, and powered water mills. In Figure 9, the course of the Wallensteingraben between Lake Schwerin (bottom) and the City of Wismar is shown. Today's remains of the artificial waterway (Goldammer, 1997) are indicated by circles, the locations of old mill races along the natural river (Hohensee, 1989) by water wheel symbols. Some remnants of the channel are still visible, though much of the canal was destroyed during railroad construction. Figure 10a shows a part of the summit reach excavated up to 10 m deep. In parts, the earthworks of the locks can still be identified, as well as stones and bricks originally used for the sluices can be found which were re-used later in other buildings, Goldammer (1997). A small natural stream which drains the hinterland of Wismar and provided fresh water, fish and water power for the local population and eventually became part of the canal.

Turbines were installed at some of the historic mill sites to produce electric energy, but only two turbines still utilize this renewable energy resource. Over the centuries a, new biological and morphodynamic equilibrium has established itself in such man-used and -made 'river' reaches. The development strategies, where weirs and mill races are removed, often mean that the historical remains of early hydraulic and cultural engineering activities are destroyed. Figure 10 illustrates the state of the old canal as it can be seen today.

Currently the Wallensteingraben is used for flood control and to stabilize the water level in Lake Schwerin. The average discharge at the northern outflow of the lake is about 0.8 m3/s. Dating back to the times of the former GDR, parts of the river are designated Nature Preservation Areas. Concerning the channel, plans are currently emerging to again connect Lake Schwerin to the Baltic Sea, thus, to re-activate or rather rebuild the canal along a new, slightly changed course not for the transport of salt, but for leisure boating and recreation to further the economic development of the region once more (Buga, 2000)
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A study of some small rivers in Central Europe indicated that these water courses have been altered by man for many centuries for varying purposes ranging from power production over transport to fortification. One additional aspect of the changes often was the reduction of the gradient and therefore the dynamics of the river. The geometry and functionality of our small rivers can therefore not be understood without their historical context. The ecosystems which evolved around the rivers are subsequently adjusted to an 'artificial' situation, but have sometimes been established for a very long time. Although the ideal aim of a 'natural river' is under discussion, it appears that in today's world the constraints imposed by infrastructure and settlement are too severe to allow for a river to develop its own natural state again.

The removal of all historical artefacts which usually is a consequence of re-naturalization deprives our society of a part of its cultural heritage and therefore of a part of its identity. In Canada, this has been realized some time ago and the Canadian Government has designated a number of river sections as 'Heritage Rivers', which are protected river sections of (amongst others) special ecological, geological, archeological, historical or recreational interest. The aim is to systematically create and maintain a Canadian River Heritage so that Canada's nature, history and society is reflected in the river system without which the development of the country had been impossible. Within the Canadian River Heritage System (CHRS) these sections are administrated and managed, Nagel & Goldhammer (1997). This system by now comprises nearly 6000 km of rivers, amongst them a long stretch of the Grand River. CHRS incorporates a large variety of nearly 100 organizations ranging from Universities to representatives of the indigeneous people (Six Nations Council), cities, government authorities, newspapers etc. which all have an interest in their river. It appears that the idea of the 'cultivized landscape', as a system not just with hydraulic and environmental aspects but also with a historical context which is worth preserving has been taken to a practical conclusion by CRHS and may serve as a model for further developments in other countries.

Re-naturalization is today considered to provide a natural environment for the development of an ecosystem as well as an area for recreation. The inclusion of the cultural heritage in the management of small rivers today would imply that the river would have to retain some usage apart from recreation. The development of rational strategies of usage of the available power and water which are compatible with the historical usage could present a new area of activity. Some progress in the field of power production has been made recently, where it could be shown that water wheels are not the out-of-date inefficient machines they are usually considered to be. A detailed literature review revealed that water wheels can be regarded as very efficient and ecologically acceptable energy converters for low head hydro power conversion, Müller & Kauppert (2003). This means that even our small rivers can - as renewable energy sources - contribute to the overall aim of carbon-dioxide reduction.

The authors are convinced that the re-naturalization in particular of canalized sections of small rivers is of great benefit to our environment. The re-naturalization of rivers which are still in a state where the old usage is recognizable (see section 3.2) would however destroy the cultural heritage. With the inclusion of the historical context, and the preservation and purposeful re-use of the resource river as e.g. a renewable energy source, it becomes possible to preserve our cultural heritage as well as to develop a functioning eco-system (influenced by man of course to some degree) around our small rivers. In a re-naturalization (or river restoration) project therefore the 'added value for nature' of the restored river and the 'historical value' of the river (and a possible further use by man within the context given by history) should be weighed against each other in order to obtain an acceptable solution. An existing water mill e.g. could be protected and, preferably, be operated again and integrated into an overall solution. A very small old canal which can never be used again however, may currently only provide a poor natural environment and should therefore be 'restored' or rather upgraded to create a functioning and ecologically diverse solution with the emphasis on the ecosystem rather than the historical value. In conclusion it can be said that the 'cultivized landscape' as perceived by engineers should be expanded to include another dimension, namely time.

>From the Middle Ages onwards the small rivers in Central Europe were re-built and used by man for a variety of purposes. Before the 20th Century, the small rivers formed a vital part of the economy. The remains of these activities are still in existence, but often not even recognized as such for lack of knowledge and awareness. Examples for such remains are:

In addition, some old artificial waterways built for transport are today considered as being natural. Past and current re-development strategies for small rivers (canalization and re-naturalization) very often neglect the historical context and therefore result in the loss of historical artifacts and a diminution of our cultural heritage. An integrated solution for the development of small rivers which includes and weighs ecological as well as cultural aspects, and possibly incorporates a re-use of the historical installations such as micro hydropower, should therefore be sought. The scope of an engineering evaluation of our environment or ‚Kulturlandschaft' should be expanded to include another dimension, namely time.