The return of degraded stream ecosystems by using positiveimpacts from near-natural sections: a new practical guidefor restorationsGeorg Gellert, Stefan Behrens & Monika Raschke

North Rhine-Westphalia State Agency for Nature, Environment and Consumer Protection, Recklinghausen, Germany

Keywordsfish fauna; hydromorphology; invertebrates;

migration; recolonization; running waters;

spreading effect; stream restoration.

CorrespondenceGeorg Gellert, North Rhine-Westphalia State

Agency for Nature, Environment and Consumer

Protection, Postfach 101052, 45610

Recklinghausen, Germany. Email:

georg.gellert@lanuv.nrw.de

doi:10.1111/j.1747-6593.2012.00307.x

Abstract

Streams in North Rhine-Westphalia (Germany) are frequently impacted by hydro-morphological degradation leading to a loss of sensitive species and a reduction indiversity. These findings mean that the final objective of the European Water Frame-work Directive to achieve the good ecological status will be failed for a large numberof streams. In a holistic approach, a new concept, considering the cost efficiency,has been developed to enhance habitats by morphological changes. This guideserves as a basis for stream restoration projects. The theoretical background is thatnear-natural sections influence adjacent sections of medium habitat quality posi-tively by migration of type-specific sensitive organisms. The preconditions are alocal taxa richness in the catchment area, the reduction of nutrients and organicmatter, and the guarantee of free movement. This so-called ‘spreading effect’ canbe measured upstream and downstream. The length of by these means valourizedsections depends on the stream type and on the biological quality component.

Introduction

The majority of rivers in North Rhine-Westphalia (a federalstate in Germany) are fragmented and morphologically in apoor condition. The physical habitat degradations are themost important pressures and the remaining threats tothe aquatic biodiversity. Nevertheless, the good ecologicalstatus, as a general quality target demanded by the EuropeanCommission EU Water Framework Directive, must beachieved. It is determined by the biological quality compo-nents listed in Annex V.

It is known that near-natural sections of running watershave a positive impact on neighbouring, structurally alteredsections, thus causing an instream habitat enhancement(DRL 2008, 2009). This phenomenon was introduced in NorthRhine-Westphalia as ‘spreading effect’. It is based on theactive and passive migration of fauna and flora from thesesections into the water surroundings. Thus, upstream anddownstream morphology has a significant influence on thelocal ecological status. The knowledge and utilization ofthese effects is the basic idea to restructure streams in aconsiderably more cost-efficient way.

Therefore, a project has been started to develop thiscompensatory concept called ‘Strahlwirkungs- undTrittsteinkonzept in der Planungspraxis’ (Landesamt fürNatur, Umwelt und Verbraucherschutz des Landes Nordrhein-Westfalen; LANUV NRW 2011a) to operationalize this ‘spread-

ing effect’. It is published as a practical guide for restorations.North Rhine-Westphalia is now following new paths in regionalrestoration programmes, which require a holistic look at thecatchment area and designs at a regional level.

Fundamental principles

‘Spreading effects’

The ‘spreading effect’ defined for natural water bodies andintroduced as term by Kail & Hering (2009) is the maximumdistance at which near-natural conditions produce a signifi-cant positive compensatory influence over adjacent sectionsof lower ecological and morphological qualities. It is based ontwo mechanisms:(1) An active and passive migration of aquatic fauna and florafrom adjacent sections of good ecological status into thewater surroundings.(2) An overlapping of degraded downstream reaches byfavourable conditions like type-specific water temperatureand sediments from adjacent near-natural sections.

Hydromorphology

The river habitat survey follows the German method ‘LAWA-vor-Ort’ (LAWA 1998). It is designed for small to mid-sizedwatercourses with bed widths between 1 and 10 m and

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visible bottom. Twenty-five parameters are recorded for100-m channel segments and compared with the state thatwould be established itself after abandoning all existent uses.There are seven morphological structural classes betweenunchanged (structural class 1) and completely changed(structural class 7), as shown in Table 1 following Raven et al.(2007).

Function elements

The requirements for the function elements given below areintended for natural water bodies. Their application in con-junction with heavily modified water bodies (HMWBs) shouldbe based on the same metrics as for natural water bodies.But, the quality targets should be adapted on a case-by-casebasis in dependence of inevitable pressures, like urban areas,flood control, navigation, hydropower production or drinkingwater reservoirs.

The function elements of this concept are:

• basic sections,

• valourized sections,

• stepping stones, and

• transit sections

Basic sections

Basic sections are near-natural sections with reference con-ditions reflecting minimal anthropogenic disturbance havinga positive impact on neighbouring, structurally altered sec-tions of running waters. Ideally, these high-quality habitatsproduce a surplus of type-specific species that colonize tem-porarily adjacent sections of poorer morphological qualitywith an affected fauna.

If it is not possible to identify, if a structurally undisturbedsection is colonized by type-specific species or only exports

favourable abiotic conditions, the good morphologicalquality is used as a criterion for identification. The presenceof basic sections with a type-specific biota corresponding tothe stream type must be served as a criterion for prioritiza-tion of restoration processes. It is appropriate to start resto-ration projects from basic sections.

In Table 2, the requirements for the morphological qualityfor the basic sections are described for natural water bodies,independent of the stream type. Basic sections should beclassified in the best tree morphological categories, and it isfavourable when this classification will be found coherently.

Table 3 shows requirements for the minimum length ofbasic sections to enhance the ecological status in naturalwater bodies, according to Koenzen et al. 2008. The require-ments depend on the stream type (small-to-mid-sized, mid-sized-to-large streams) but not on the region (lower mountainand lowland areas). It is important to point out that thedemanded hydromorphological quality must be coherent.

Valourized sections

Valourized sections are medium-impacted by morphologicalstressors (structural classes between 1 and 5 for river bedand banks) and have an affected aquatic fauna. The valouri-zation effect is defined as the active or passive dispersion oftype-specific species from adjacent basic sections in reacheswith medium morphological qualities for at least a tempo-rary colonization. This phenomenon leads to a faunisticrecovery of valourized sections. The range of these sectionshas been determined as the maximum length at which aminimal positive biological response could be statisticallyproved by neighbouring sections of good morphologicalqualities. Valourized sections are targets for restorationactions, if the demanded requirements (Table 4) are notfulfilled.

Table 1 The morphological classification of rivers in Germany

Structural

class

Description of

morphological structure Modification within river reach

1 Unchanged Matches the potential natural state

2 Slightly changed Slightly influenced by localized structures

3 Moderately changed Moderately influenced by multiple, localized interventions

4 Distinctly changed Distinctly influenced by different interventions

5 Obviously changed Obviously influenced by extensive interventions like culverts, elongated embankments, small substrate

diversity, presence of artificial material, local bank fixation and lack of vegetation, non-natural land-use

6 Strongly changed As (5) but very negatively influenced

7 Completely changed As (5) but completely negative influenced

Table 2 Requirements for basic sections – morphological quality

Stream types

River bed (fishes and

macro-invertebrates)

Banks (fishes and

macro-invertebrates)

Riparian zones (fishes and

macro-invertebrates)

All stream types Structural class 1–3 Structural class 1–3 Structural class 1–3

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The requirements for the morphological quality are differ-entiated between the stream types and the biological qualitycomponents. They are only described for natural waterbodies. In case of HMWBs, the ‘spreading effect’ decreases;nevertheless, the good ecological potential can be arrived inindividual cases. For HMWBs, the basic conditions are theabsence of backwater and only a slight disturbance of theriver continuity.

The range of positive impacts of near-natural habitats(basic sections) was defined on a direct statistical approachusing own monitoring data and data from the literature.Details are published in Landesamt für Natur, Umwelt undVerbraucherschutz des Landes Nordrhein-Westfalen [LANUVNRW] (2011b).

The valourizing effect loses its influence as the distanceincreases. In Table 5, the maximum ranges are listed, whichcan be achieved (depending on the stream type and the bio-logical quality component) under the described conditionsand supported by stepping stones.

Stepping stones

Stepping stones are small structurally rich sections withgood habitat properties (e.g. accumulation of deadwood,

aquatic plants) that can be populated at least temporarilyin order to enhance habitat connectivity. They lengthenthe spreading positive effect of migration in valourizedsections.

Transit sections

Transit sections only have a passing function and do notpermit colonization but enable the migration between adja-cent basic sections. The only requirements are the presenceof a near-natural bed substrate and continuity.

Table 6 shows the maximum possible length of transit sec-tions for the biological quality components. The maximallength of the transit sections for the fish fauna is the distancesumming up the lengths of the transit sections upstream anddownstream.

Among valourized sections, the transit sections should notexceed the determined maximum distance; otherwise, theyare unable to serve as passageway.

Figure 1 shows an exemplary sequence of existing and tobe developed function elements taking a small stream in thelower mountain region as example. Where the promotion ofbiodiversity is no longer detectable downstream, a new basicsection must be established.

Table 3 Requirements for basic sections – length

Stream types Length of basic sections (Fishes and Invertebrates)

Small-to-mid-sized streams in lowlands and lower mountain areas At least 500 m

Mid-sized-to-large streams in lowlands and lower mountain areas At least 1000 m (catchment area < 1000 km2)

At least 2000 m (catchment area 1000–5000 km2)

At least 4000 m (catchment area 5000–10 000 km2)

Table 4 Requirements for valourized sections (including stepping stones) – morphological quality

Stream types

River bed/banks (fishes and

macro-invertebrates)

Riparian zones

Fishes Macro-invertebrates

Small-to-mid-sized streams in lowlands and lower mountain areas Structural class � 5 No requirements Marginal zone existing

Mid-sized-to-large streams in lowlands and lower mountain areas Structural class � 6 No requirements

Table 5 Range of valourization (including stepping stones) for small, mid-sized and large streams in lowlands and lower mountain areas

Stream types Fishes Macro-invertebrates

Small-to-mid-sized streams in lower mountain

areas

As long as the bordering basic section, max.

3500 ma

As long as the bordering basic section, max.

2500 m

Mid-sized-to-large streams in lower mountain

areas

As long as the bordering basic section, max.

4500 ma

As long as the bordering basic section, max.

3000 m

Small-to-mid-sized streams in lowland areas As long as the bordering basic section, max.

3000 ma

Half the length of the bordering basic section,

max. 1000 m

Mid-sized-to-large streams in lowland areas As long as the bordering basic section, max.

4500 ma

Half the length of the bordering basic section,

max. 2000 m

aMax. lengths of the valourized sections for the fish fauna is the result of the summation of the reaches upstream and downstream.

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Besides the described functional elements, alsodegraded sections can be found in a watershed context.They can not be valourized and are of poorer quality thantransit sections, like pipings or backwaters. Degraded sec-tions do not belong to the basic elements of the ‘spreadingeffect’. They are remaining reaches in a planning area, whichcould not fulfill one of the demanded needs for a transitsection because of restrictions. These sections should besignificantly reduced.

Biological components

Macro-invertebrates

The macro-invertebrate fauna can especially be regarded asthe result of the morphological quality of the site. The assess-ment of the migration range in the lower mountain areas isbased on literature data from Kail (2009), Koenzen et al.(2008) and Schattmann (2008). The reach of migration effectsdownstream depends on the adjacent length of the basicsection but does not exceed 3000 m (Table 5). The migrationupstream has not been quantified in this project.

The assessment of the length of positive effects instreams in lowland areas is based on literature data and onown monitoring data (LANUV NRW 2011b). Statistical analy-sis of these data leaded to the conclusion that basic sectionsinfluence positively the biodiversity of downstream sectionsonly at the half of their length not exceeding 1000 m for

small-to-mid-sized streams and 2000 m for mid-sized-to-large streams (Table 5).

Apparently, the maximum ‘spreading’ range for macro-invertebrates is greater in the lower mountain areas than inthe lowlands. Obviously, the migration caused by drift playsan important role, which corresponds to the current state ofknowledge. But, the drift is not the only determining route ofdispersal. Monitoring data, which were fortunately gained 7weeks after the establishment of a bypass channel with alength of 2.6 km in the river Lippe (North Rhine-Westphalia),collected after a standardized river assessment methodfor Germany called ‘Perlodes’ (Meier et al. 2006), showedamazing results (Gellert 2011).

In Table 7, it is shown, that the dispersal was predomi-nantly upstream for six of nine species. For Gammarus pulex(crustacean), Baetis vernus (ephemeropteran) and the tri-chopteran genus Hydropsyche, only an upstream movementcould be noted. Because in upper reaches colonies of thesetaxa were registered, a downstream dispersal would havebeen noticed.

Also, the ephemeropteran species Caenis rivulorum andC. luctuosa migrated actively upstream. They could equallybe displaced downstream by the current (drift). But, thiscourse could not be noted because they were absent inupper reaches. The ephemeropteran B. scambus showsgreater activity upstream. Simuliidae (blackflies) and thecase-building caddisflies Lepidostoma hirtum and Hydroptilasp. were only dispersed by the drift because of the case,

Table 6 Requirements for transit sections – length

Stream types Fishes Macro-invertebrates

Small-to-mid-sized streams in lower mountain

areas

One quarter of the length of the basic section

and max. 900 ma

One quarter of the length of the basic section

and max. 600 m

Mid-sized-to-large streams in lower mountain

areas

One quarter of the length of the basic section

and max. 1200 ma

One quarter of the length of the basic section

and max. 700 m

Small-to-mid-sized streams in lowland areas One quarter of the length of the basic section

and max. 900 ma

One quarter of the length of the basic section

and max. 600 m

Mid-sized-to-large streams in lowland areas One quarter of the length of the basic section

and max. 1200 ma

One quarter of the length of the basic section

and max. 1200 m

aMax. lengths of the transit sections for the fish fauna is the result of the summation of the reaches upstream and downstream.

Fig. 1. Schematic view of the function elements using ‘spreading effects’ to adjacent sections with an affected fauna (figure following DRL 2008).

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which limits moving activities. An upstream and downstreamdispersal showed the crustacean Echinogammarus berilloni,the ephemeropterans B. rhodani, Serratella ignita andspecies of the family Chironomidae.

For over 20 years, upstream movement instream wasregarded as a distinct dispersal mechanism and an importantfactor of recolonization (Söderström 1987). Recent studies byElliott (2003) revealed that dispersal in upper reaches waspredominantly for many invertebrate species, the exceptionbeing the case-building caddisflies.

It can be assumed that the number of taxa, which migratesupstream, is high. Lotic animals are positively rheotactic andtend to crawl against the current. In the lower mountainregions, a distance up to 1 km can be travelled upstream bymany invertebrate species. Consequently, dispersal instreamis not only passive. These findings support the scientificbases of this concept.

Fish fauna

The range of positive impacts of basic sections in streams inthe lower mountain areas for the fish fauna was statisticallydetermined on the basis of reference data (Koenzen et al.2008; Schütz et al. 2008) and on monitoring data (LANUVNRW 2011b). The range of effects (depending on the lengthof the adjacent basic section) is up to 2500 m downstreamand 500 m shorter upstream (Table 5).

The assessment of the length of positive effects in streamsin lowland areas is based on own evaluations, on monitoringdata and on reference data (LANUV NRW 2011b). The dataanalysis showed positive impacts downstream and upstreamequally on a similar scale.

Macrophytes

The estimated range of positive impacts for macrophytes bybasic sections in streams in the lower mountain areas and inlowland areas is only based on reference data and on expertjudgement (LANUV NRW 2011b). A positive effect is only

downstream possible. A minimum length of valourizingeffects could not be estimated.

Additional requirements

To enable restoration success, the described requirementsfor the sequence and design of basic and valourized sectionsonly have the beneficial effects, if all the following frameworkconditions are fulfilled with equal priority:

• No temporal drying up.

• Water quality corresponds to the good chemical status.

• No significant organic pollution.

• Thermal conditions close to natural conditions.

• Sediment, hydrological and hydraulic conditions are com-pliant with the corresponding stream type.

• Local richness of type specific organisms as recolonizationpotential in the entire catchment area.

• Habitat reconnection.An ideal distribution of function elements within a planningcatchment can be recommended to produce positivechanges in the biotic composition. Added up, macro-invertebrates need between 40 and 50% of the water systemas basic sections, while the type-specific fish fauna is con-tented with 25% of the stream system with near-naturalreference conditions. Generally, transit sections should notexceed 15%, and degraded sections should not exceed 10% ofa stream system.

The time span to reach the objectives varies considerably.The larger the number of type-specific species in the planningcatchment is, the shorter is the period of achieving the goodecological status. But in case they have been brought toextinction in the whole basin, the process may operate intimescales of decades or longer (Hering et al. 2010).

Even HMWBs or ‘rigid restricted areas’ can potentially bemade accessible for the instream habitat enhancement bycreating or protecting basic sections and by introducingelements for the spreading pathway within the restrictedarea. But from the catalogue, all those measures must beexcluded, which impede existing uses.

Table 7 Direction of the migration of selected invertebrates

Taxa

Dispersal distance in m Presence

Upstream Downstream Above bypass Below bypass

Gammarus pulex 550 yes yes

Baetis scambus 550 150 yes yes

Baetis vernus 550 yes yes

Caenis rivulorum 150 no yes

Caenis luctuosa 550 no yes

Hydroptila sp. 650 yes yes

Hydropsyche sp. 850 yes yes

Lepidostoma hirtum 850 yes yes

Simulium sp. 1150 yes yes

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To conduct a restoration project, it is necessary to follow ahierarchical strategy of activities. After identification of basicsections in the entire fluvial system, valourized sections canbe established between them.

Planning example

Figure 2 shows a fictitious sequence of existing and still to bedesigned function elements taking a small stream system in alower mountain area as a river basin management plan.

In valourized sections where there are restrictions causedby urban areas, the range of vitalization is shorter (stream Ain an urban area). In these cases, the next basic section down-stream should follow in a shorter distance (stream A belowthe urban area).

Within larger valourized sections where the possiblemaximum range of vitalization has not been achieved, step-ping stones of the ecological quality of basic sections (butshorter in the length) should be planned (stream C). Theincrease of local physical heterogeneity promotes the condi-tions for type-specific organisms.

The stream crossing structures in the streams A and Cmust be removed. But, the dam of the reservoir in the river B

remains as a barrier for migrating organisms. In such cases, abasic section must be planned in direct proximity down-stream to this restriction to minimize negative impacts fromthe reservoir and to rehabilitate downstream reaches.

Conclusions

(1) The migration of macro-invertebrates, fishes and macro-phytes is a fact that can be used to enhance the biologicaleffectiveness after restoration activities.(2) This methodological approach is suitable to enhance eco-logical functions of natural and HMWBs in compliance withthe principle of cost-efficiency.(3) A detailed watershed assessment and restoration priori-tization is needed. Starting point for all planning are basicsections.(4) The restoration must target the most limiting factors.(5) The biotic response to restoration is less well known andpoorly predictable. It can take years or decades.

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River dam

Source

Source

Already realized restoration (stepping stone of high quality)

Already realized-restoration (basic

section)

500 m

Valorized section

Transit section

Degraded section

Existing potential basic section

Basic section to be designed

Existing stepping stone

Stepping stone to be designed

Urban area

Elimination of migration barriers

Stream

Development of a stepping stone of high quality

Stream A

Stream A

Stream B

Stream C

Fig. 2. Schematic view of the sequence of function elements in a fictive planning area taking a small stream in a lower mountain area as example.

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6 Water and Environment Journal •• (2012) ••–•• © 2012 The Authors. Water and Environment Journal © 2012 CIWEM.

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