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Author(s): Rolf Bostelmann
Editorial office: FVA, Germany
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Ecological function of small watercourses

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Der Ooser Landgraben bei Rastatt
Fig. 1: The Ooser land ditch near Rastatt – it’s a small stream in the forest, but it’s not a forest stream.

What are small streams?

Small streams are classified as streams which are approx. 5 m to max. 10 m wide. The difference between streams and rivers is not defined as in reality the transition is smooth. The rule of thumb is a stream turns into a river when the riparian vegetation can not develop a canopy closure above the river bed.

‘Small’ streams make up the largest share of the water system. Briem (2003) declares that small streams represent more then 90 % of the entire length of all streams.

What are small streams in the forest?

When we focus on small streams in forests, we should keep in mind that we are focussing only on one part of the stream system - a small part that stretches across the landscape like a patchwork quilt and is indeed remarkable. The length of small streams in forests in Baden-Württemberg has been estimated as 15,500 km.

Small streams in forests are not automatically semi-natural forest streams (see Fig. 1). The forest is no garden of eden in which everything is perfect. It is a place of production just like the rest of the landscape. It’s hardly surprising that the production hasn’t also influenced forest streams. Small streams in forests may represent all states of degradation from semi-natural to severely degraded. The location in the forest is no guarantee of stream naturalness. But if there are natural streams they will probably be found in the forest.

The following article focuses on semi-natural forest streams. The changes in forest streams due to forestry and other human impacts is discussed in the rest of the paper.

Function and importance of small streams in forests

The question of function and importance of small streams is a large topic, which cannot be discussed within the scope of this article. However, two perspectives can be distinguished:

  • How do water bodies, flows and aquatic communities interact and how does this create so many diverse streams? This perspective considers the "internal" functional structure of streams.
  • What is the function of small streams in the landscape? Here the focus is on landscape-ecological aspects.

Further information on small streams is listed at the end of this article

Running water – the formative power of our landscape

Two basic functions of streams – small or large, in the forest or in the open landscape – have to be acknowledged:

  • Streams are an essential element in the water circle and with their wide network, are an essential component of the water balance.
  • The power of running water forms not only streams and rivers but also valleys and flood plains. Our entire landscape is formed by running water – even when we hardly recognize the process, which occurs over unobservable timescales.

Next to these large-scale and slow landscape changes, running water has effects at a small-scale and over noticeable periods. The flow creates scours and baffles, causes banks to collapse, differentiates bed material into various particle sizes and changes the mosaic of habitat niches. The flow develops the diversity of the water body structure and subjects the structure to a continuous spatial and temporal change regime. The structural diversity and dynamics of semi-natural streams and flood plains are not imaginable without running water.

The impact of water as a creating power over different time and spatial scales is shown in figure 2.

Space-time-levels of change in streams
Fig. 2: Space-time-levels of change in streams (Principle presentation from Graw & Borchardt 1999 after Frissel et al. 1986).

Longitudinal classification of streams

Every natural stream changes its characteristic form on its way from the spring to the stream outlet resulting in the rough, well known classification: spring, upper reach, middle reach, lower reach and stream outlet (see fig. 3).

Basic longitudinal classification of streams

Fig. 3: Basic longitudinal classification of streams (from Graw & Borchardt 1999).

- Click to enlarge -

The longitudinal change of a stream not only affects physiographic factors such as fall, flow, bed material load and stream form but it shapes and changes the biocoenosis of the stream as well.

Stream organisms have in the course of their development adjusted to the specific living conditions of the varying stream zones and sort of "learned" to exist there. As a result, many varying water biocoenosis have developed, which replace each other continuously along the stream. These changes are so characteristic that it is possible to separate a natural stream into different biocenotic zones or stream regions. A well known classification system is based on keystone fish species.

For our small forest streams the spring, upper and middle reach, thereby including the upper and lower trout-regions, are essential. The keystone fish of these zones are brown trout, common minnow and European bullhead.

Idealised stream system and its longitudinal classification
Fig. 4: Idealised stream system and its longitudinal classification.

The different reaches of a stream merge more or less continuously into each other from the spring to the outlet and are connected through running water, mass transportation and aquatic organisms. They belong together as a whole or continuum.

Small streams

Springs

Due to their strong groundwater impacts, springs have specific living conditions and create their own independent biotope. Their biocoenosis - the so called eucrenon - is characterised by many highly specialised animal and plant species, many of which are endangered.

Springs flow into small rivulets, which are largely characterised by groundwater. Therefore they feature many spring-like characteristics. This also applies to the colonisation of spring rivulets, which are counted as a spring biotope (Hypocrenon).

Quellrinnsal inmitten einer Quellflur
Fig. 5: Spring rivulet within a meadow in the Buntsandstein-Odenwald.

The transition from a spring rivulet to a small stream

The strong groundwater influence decreases with increasing distance from the spring and spring rivulets transform smoothly into small stream upper reaches. Whereas springs rivulets are solely, or to a large extent solely, groundwater-fed, surface runoff gains more importance to channel flows with increasing distance from the spring. This doesn’t mean only that the channel flow (and therefore the stream) increases but that it becomes more unbalanced. The flowing water – and especially high flows - is becoming a formative power and an ecological determining factor. Both the water body structure and water biocoenosis are increasingly determined by the flow and its dynamic.

The stream zone that follows the real spring region (crenon) is called rhitral. It is characterised by balanced cold water temperatures in summer and a continuous high oxygen content.

The small streams of this transition category are scarcely half a meter wide (see fig. 6). But they represent an extraordinarily large portion of the entire length of the water system. They create a wide network into remote reaches of our landscape. These often unimpressive streams have an important value for the water balance as well as for the ecosystem. This particular importance has been disregarded in the past.

Mountain streams

Most forest streams of Baden-Württemberg belong to the mountain stream category because the largest contiguous forests are in the low mountain ranges. The following considerations apply to small-scale water body structures.

Typical water body structures

Flow structure

On its way from the mountains to the plains water assumes a different form of flow.

Ein kleiner Nebenbach des Ehrenstetter Ahbaches im Südschwarzwald.   Bachbett
Fig. 6: A small tributary stream of the Ehrenstetter Ahbach in the Southern Black Forest. The left picture shows a small stream embedded in its accompanying riparian, the right picture its bed.

In the sloping upper reaches of mountain streams, water falls in small waterfalls and spillway chutes down to the valley, which can be seen as cascades (see fig. 7).

Oberer Zastler
Fig. 7: Upper Zastler below the Zastler Cirque at the Feldberg in the Black Forest.

A cascade is composed of a small natural fall of boulders or solid rock followed by a pool, which the water falls into. These natural pools can be very deep or – in another extreme - very shallow and wide. Water is collected here and then falls at the next small fall. In this way the water constantly alternates between extreme flow conditions, plunging and roaring on one hand then sedate circling in scours, pools and backwaters. Despite the large fall and movement the water moves paradoxically relatively slowly. Accompanying this type of flow is a massive energy change – a circumstance that is important to the water body structure and the stream inhabitants.

The close spatial interaction of extreme flow structures is the reason for the extraordinary structural diversity of streams. Alongwith bare, smoothed rock and large boulders the stream bed contains particles of all sizes including fine material such as sand. Even accumulations of leaves and other organic material belong to the inventory of small structures and small habitats.

As the slope decreases, the cascade-like flow changes gradually into a sequence of pools and riffles (see fig. 8). In riffles the water is shallow and flows quickly over coarse bed material, in pools the water is deep and flows slowly.

Wechsel von Stillen und Rauschen in einem Bergbach
Abb. 8: Change of pools and riffles in a mountain stream (according to Otto 1991).

This almost rhythmic change of the flow structures is based on a natural longitudinal classification of the stream bed. Its development is related to a complicated interaction of different factors which has been barely researched. An intact stream bed longitudinal classification (noticeable through an almost periodic sequence of pools and riffles) is therefore an important characteristic of a semi-natural stream of low mountain ranges.

Bed structures

Semi-natural streams in low mountain ranges have markedly uneven cross sections which vary in width and depth depending on the bed classification, the bed load, the stream form and the bank vegetation.

The stream bed has a diverse structure. It’s not seldom that it appears as a chaotic mosaic which conceals that its structure is subject to a regulative principle: the flowing water.

The following interconnected factors have a great influence on the development of and temporal-spatial change in bed structures:

Naturnaher Bergbach im Keuper-Bergland
Fig. 9: Semi-natural mountain stream in the Keuper mountain reaches – the Brettach near the town of Brettach.
  1. Firstly the natural longitudinal classification of the stream bed, as well as the related flow structures, are important structure determining factors.
  2. Additionally the stream bed paving plays an important role. This bed is made up of dense and closely joined rough paving material and gives the stream bed a relative stability, including during times of high water runoff. Natural paving is a typical characteristic of many semi-natural streams in low mountain ranges. However certain stream types do not form a natural paving, or only in fragments e.g. torrents or sandy streams.
  3. Another essential factor of the bed structure is the bed load discharge. While fine bed load material such as sand is continuously (also during times of normal runoffs) transported, rougher material only moves downstream in batches during runoff peaks. If the rougher bed load discharge is set in motion during high water, it, is sorted into different particle sizes as it moves along the bed and comes to rest with decreasing flows. Thereby different smaller or bigger sized sedimentations are formed. They create a differentiated structure mosaic, which is relatively stable until the next high water.

High water does not only form the stream bed but also the bank through lateral erosion and accumulation. It washes out banks, forms small islands, forms new banks or destroys them.

An overview of stream splitting and islands in the Reisenbach stream in the Baden Buntsandstein-Odenwald gives an impression of the large diversity of forms (see fig. 10).

Forms of stream splitting in a small mountain steam in the Buntsandstein-Odenwald

Fig. 10: Forms of stream splitting in a small mountain steam in the Buntsandstein-Odenwald (Nadolny et al. 1990).

- Click to enlarge -

The biocoenosis

Fauna

The flowing waters of mountain streams with their strong currents are extreme habitats which only a few species with special adaptations can colonise. In the flowing water itself only a few animal species can exist especially fish. First of all we have the brown trout which colonises the cool upper reaches, often as the only fish species present. This gives this stream region its biological name "trout region". Further downstream – in the lower trout region – the brown trout are often accompanied by the European bullhead, common minnow and more rarely Brook lamprey. When the grayling appears, the "grayling region" begins and signals that the stream is changing into a small river.

Unlike fish, there are no invertebrate species which are able to permanently colonise the free water body of a mountain stream. The flow would carry these animals away. The invertebrates of mountain streams are dependent on a compact base such as stones or branches which the can latch or stick onto. These species developed special physical and behavioural adaptations to avoid the permanent danger of drifting away. Some animals hold on with suckers to stones, others occupy the no flow zone behind stones or coarse woody debris. Some species have an extremely flattened body which offers no possibility of being carried away by the flow.

Life cycle of the brown trout
Fig. 11: Life cycle of the brown trout (from Bostelmann 2003).

An important habitat of steams in low mountain ranges is invisible to the human eye but is still of fundamental importance. This habitat consists of the loose and permanent gravel just below the stream bed - the interstitial. Due to the balanced temperature and flow conditions living conditions are much better than in the flowing water body. The stream bed and interstitial region below represent the most densely populated habitat of a mountain stream. A large number of animal species spend their sensitive juvenile stage in this hidden gap system beneath the stream bed. Examples are brown trout, grayling and others who spawn into the gravel (see fig. 11).

Despite their extreme living conditions, mountain streams contain extraordinarily species-rich habitats. The key to this diversity is its large structural richness, which is biological seen as the determining factor for the number and mosaic of colonisable niches or habitats. Due to nutritient requirements or other adaptations, many species have specific ties to certain habitats within "mountain streams". Figure 12 shows a section of a mountain stream with its typical mosaic of substrates respectively habitats and their specific inhabitants by way of example.

Flora

Higher water plants are missing in the fast flowing turbulent upper reaches of mountain streams. It is only possible for isolated water plants to colonise in calm waters and in areas which are protected from bed load motion. Not until the lower trout and grayling regions are reached can some water plants succeed in becoming a constant part of the water biocoenosis. Most of them are special flow-adapted plants such as intermediate water starwort (Callitriche hamulata), pond water-crowfoot (Ranunculus peltatus) or water parsnip (Berula erecta f. submersa).

Completely different are the mosses and algae. They form an inherent part of many mountain streams. Lime-poor streams (silicate –streams) in particular often have a rich, stream-specific moss vegetation. They form important small niches, which enriches the mosaic of small habitats.

Another important group are the diatoms, who belong to the unicellular plants. They form species-rich communities and cover the bed substrate with a thin brownish crust. They are literally grazed by many stream inhabitants and have an important nutrition function for invertebrate animals.

Habitat stream bed: mosaic of small habitats and their typical inhabitants

Fig. 12: Habitat stream bed: mosaic of small habitats and their typical inhabitants. (from Borstelmann 2003).

- Click to enlarge -

Bank borders

The banks as well as the stream bed are influenced by the dynamic of the changing discharge and the associated sedimentation and erosion processes. They are also characterised by stream specific biocoenosis. Of most importance is the stream-alder-ash forest (Stellario-Alnetum glutinosae). This forest is well developed along larger meadow streams, which forms in valley bottoms. The stream-alder-ash forest is able to form very dense and closed stands and therefore is a characterising landscape element of meadow valleys.

We all know this wonderful picture – and when we go with this (idealised) picture into the forest and look for a comparable stream-alder-ash forest, disillusion often follows:

If we find a steam-alder-ash forest at all than it is markedly open, a small strip that – when we look at the tree layer – seems to be interrupted again and again. This has several reasons: natural and due to utilisation. The natural reasons depend on the competition between the stream-alder-ash forest and the neighbouring forest. Furthermore, the stream-alder-ash forest is a very low productive forest; therefore a type of forest that was not supported in the past. It was at the most tolerated and therefore seldom occurs.

What then does a natural stream-accompanying riparian forest along a natural stream in a natural forest look like? This question needs further surveys before it can be answered in all aspects.

Every stream – how large or small it is – has its own stream specific riparian margin. This is an irreplaceable part of a natural stream.

The pivotal questions are: How wide is a typical riparian margin? What is it composed of? To this question Bönecke (2004) provides a pragmatic answer.

The following are some aspects of why intact riparians forests have such an outstanding importance for small streams.

  • Support diverse, structural rich bank formations and therewith the stream bed. Development of typical stream banks and small structures e.g. flooded roots as refuge for trout.
  • Deposition of foliage – especially from black alder - as an essential nutrition for a group of organisms (e.g. freshwater shrimp)
  • Deposition of fallen wood as colonisable substrate. This is especially important in slow flowing streams with fine sediment-rich bed load materials.
  • Deposition of large branches and trunks (coarse woody debris) as structure forming materials and supporting further development at the same time.
  • Due to shadowing reduces warming during summers, thereby produces balanced oxygen-conditions in the water.
  • Riparian small woods act as perches or as orientation points for upstream-oriented flying water-insects.

Regional stream types in Baden-Württemberg

Flowing waters have an intense interaction with the ambient landscape. This applies even more the smaller they are – especially for streams.

Due to the strong imprinting through their basin streams are a reflection of their landscape. Or in other words: Every landscape has its own, regional specific streams. Depending on the landscape characteristics – especially on the geology, relief, altitude and climate – each stream has its own characteristic water body structures and biocoenosis. Caused by this specific imprinting, it is possible to categorise certain macrochores (here water body landscapes) into regional stream types (see research group Fließgewässer 1998).

Due to its various landscape formations, Baden-Württemberg has a large variety of different (regional) stream types. Altogether, 11 stream water landscapes with different regional stream characteristics are distinguished. Briem (1999) described the morphological structures of these water body types especially at higher scales.

On the importance of small streams in the forest

Flowing waters – networked linear systems within the landscape

Natural steams are generally biologically continuous – their inhabitants passage is unhindered. This criterion not only applies to fish but also to small aquatic animals. The biological continuity is the precondition to preserving the internal functionality of the networked system "flowing water".

That the continuity of streams is not a given in the open landscape and even less so in urban areas is not news. It is surprising however, that the continuity of small forest streams is, in many cases, also not good.

Streams – lifelines of the landscape

Streams with their branching form are by far the most important network elements of our landscape. To support or rehabilitate this function seems to become more urgent, the more the landscape is carved up and separated into isolated functional areas (see § 31 Federal Nature Conservation Act).

Streams have always fulfilled important and irreplaceable network functions: In forest-dominated natural landscapes, streams and their accompanying riparian forests have been important migration and distribution corridors.

If we want to preserve or support the network function of small streams in forests we must first of all look at streams and stream accompanying riparian margins as one unit. They form a biotope complex which is a continuous, semi-natural composed band with structural links through the entire neighbouring forest. To achieve this goal Bönecke (2004) proposes some concrete suggestions.

Semi-natural forest streams – refuge of endangered species and biocoenosis

Semi-natural forest streams are often habitats for critically endangered animal and plant species. For this reason alone semi-natural streams in forests are of great importance.

As a refuge for rare and endangered species, forest streams also act as centres of redistribution and resettlement to other renaturalised streams.

Semi-natural forest streams and flood prevention

Flood prevention should not start with the construction of new polders at large rivers but at the point where runoff forms. Ultimately the innumerous small streams. At these so called sources it is important to use natural opportunities to reduce and delay the surface water runoff as well as to hold back the flood discharge.

The forest offers, in contrast to the open landscape, excellent possibilities:

  1. Conservation and development of a semi-natural forest No other type of vegetation has such a balancing impact on the water balance as a semi-natural forest (high evaporation, low surface discharge)
  2. Conservation and development of semi-natural flowing waters Support of rough-textured bed structures, e.g. toleration of coarse woody debris to slow down discharge and to support an early expansion into the area – the neighbouring riparian forest.
  3. Conservation and development of semi-natural stream accompanying riparian margins. This should often assist in reducing and delaying the discharge of small floods.

One small forest stream with a narrow riparian margin, which has been optimised according to specified criteria, is not able to bring about large effects. But many small forest streams thousands of kilometre long and their accompanying riparian margins will have an impact.

This significant potential opportunity can be realised through clever water body and riparian margin development. Thereby forest water body development can make a contribution to flood prevention. In times when climate change appears to be on its way, flood prevention seems more urgent than ever.

Semi-natural forest streams – concrete models for water body development

Without an exact knowledge about the structures and biocoenosis of semi-naturals streams is it not possible to assess degraded streams correctly nor to plan for their "renaturation".

The best way to gain the necessary knowledge is to look at the few remaining semi-natural streams in the forest.

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