Practical suggestions to determine the ecological effectiveness of passability measures

The success of passability measures, or to put it differently, their ecological effectiveness, depends on various factors. Possessing information about the quality of these factors, allows an exact evaluation of which especially promising passability measures can be carried out on which watercourses or stretches of water. To apply this in practice, the factors which assist in decision making have been collected together in a table (attachment 1). To evaluate the ecological effectiveness of the measures its best to consider as long a stretch of water as possible (several kilometres). When evaluating the ecological effectiveness of several planned measures, the priority of implementing the individual measures can also be determined. That is especially important when resources are scarce, as in this way the most ecological effective measures can be scheduled to be implemented first.

Determining the ecological effectiveness of passability measures is based on the evaluation of six factors:

  • Chemical water quality
  • Organic water quality (Saprobic)
  • Morphological-structural impacts
  • Hydrological impacts
  • Semi-naturalness of the watercourse environs
  • Distance to the nearest up-and downstream lying travel barrier

The order of the list indicates their importance. It describes the relevance of the individual factors on ecological effectiveness. Chemical and organic water quality are placed at the very top. Only there, where the chemical and organic water quality corresponds to the good conditions of reference stream values, do passability measures really make sense, according to the basic principle of "clean water first and then ensure passability". The table in attachment 1 contains a corresponding schedule in the upper section. For each factor it is stated, under which requirements passability measures are more or less appropriate. Then, the importance of individual factors is explained in greater detail.

Chemical Water quality

A poor chemical water quality in forest streams can primarily be traced back to the adverse effects of anthropogenic caused acidification (acid rain). This appears in watercourses as areas of soils with low natural buffering capacities. The anthropogenic water acidification situation has improved a little since the 1980s. Presumably due to a decrease in the introduction of acid causing air pollutants, especially sulphur dioxide. Watercourse acidification can only be influenced to a limited extent by measures at the local scale. Ameliorating effects of soil liming, can only be expected over the long-term.

Water quality

The organic water quality (Saprobic) equates in the majority of forest watercourses to the expected optimal water quality of category I-II. One has to reckon with higher load factors due to nutrient inputs where watercourses leave the forest and flow through agricultural areas. Larger stresses can also be caused by releases from fish breeding pools and discharges from households or wastewater treatment plants.

Morphological-structural and hydrological impacts

The morpholoigcal-structural stress factor deals with interventions to the watercourse’s vertical alignment, long and cross section, and lining of the stream invert and bank. Interventions to the vertical alignment and long and cross section profiles predominately arise in forests from stresses associated with forest road construction. If roads are built in the floor of V-shaped and narrow U-shaped valleys, the stream must be pushed to the side and therefore considerably reduced in width, often reshaped into a ditch. As a consequence of channel shortening and narrowing of the discharge profile, deep erosion, due to moving bottom substrates, occurs in the watercourse which can lead to the formation of high invert steps that small creatures and possibly fish, can no longer overcome.

Lining sections of forest watercourses with stone and wood occurs in association with historical watercourse use such as with the driving and rafting of timber. Also small structures for irrigating fields are found in the forest even where the unviable field has disappeared through afforestation. If the modification or even removal of such structures is planned, then the information on the restrictions to passability measures in table 1 in attachment 1 should be considered.

Fish pools lying in the main channel cause hydrological stresses by damming back water. Depending on the size of the complex, a complete change in settings, from flowing water to still water, with a resulting break in the passability for aquatic animals, can occur.

Adverse effects of semi-natural watercourse surroundings

Adverse effects from semi-natural watercourse surroundings only occasionally result in restrictions for passability measures. Measures should be deferred in forest streams if a long stretch of the watercourse is bordered on one or even both sides by forest roads and the river bottom is predominantly man-made. This also applies to longer watercourse reaches which are enclosed by pure conifer stands.

Distance to the nearest up and downstream lying travel barrier

An important factor is the location of a planned passability measure with respect to the nearest up and downstream travel barrier. This especially applies to individual measures were large scale watercourse connections come to the fore. When restoring the passability of a travel barrier which is very far away from the next up and downstream travel barrier, its value to species that travel the numerous pathways within the watercourse is important. For these species it especially important to first achieve passability at the place where the corresponding ecological effect, with respect to the maintenance and increase of a typical aquatic biological population, will be largest

Individual passability measures in lower reaches are advantageous if the upstream watercourse – in the best case scenario to its source – is barrier free. Passability measures connecting small side streams to the main channel grant juvenile fish in particular entrance to spawning and growth areas. Such measures can not be praised highly enough. Adverse effects on fish stocks could probably be reduced in many water courses simply through the improvement of access to suitable spawning and growth habitats.

If it’s established that the fish population above the travel barrier is extinct, the causes must be clarified. Water pollution from anthropogenic causes is investigated first. If anthropogenic disturbances can be ruled out to a large extent in the future, then it is of course particularly effective to enable natural immigration to the non-fish occupied stretches of water. Fish disappear naturally, especially through water reaches drying out and more rarely following extreme flooding incidents. After such catastrophes, a repopulation from the lower reaches can no longer occur because of migration obstacles.

When implementing passability measures the question is often raised, how should one deal with a watercourse system which has a large section which is more or less permanently impassable. This case would arise at a dam for example, which has no upstream passage at all. Such situations don’t play a role, except for long distance travellers, as long as the water system lying above the travel barrier has a good habitat quality due to its size. Exactly in such isolated water systems, passability takes on a large importance for the survival of the existing fish population.

The lower table in attachment 1 contains details on passability measures constraints. These relate to the protection of threatened species and preservation orders and must be observed. If there is uncertainty about crayfish or mussel abundance, one must enquire at the fisheries office, the nature protection administration or experts familiar with local species.

During planning and carrying out passability measures, regulations of the water and nature protection authorities are to be observed. Planned passability measures should be brought to the attention of the responsible water authority. If passability measures can be carried out as river maintenance works depends on the individual case. Details referring to this are in the "Passability measures – River works or River maintenance?" article, which can also be found in the Forest and Water Internet Handbook.

Examples applying the Decision Making Assistance

The following two examples illustrate again how one can determine, with the aid of the decision making tool, on which watercourse or stretch, passability measures are sensible and on which, not yet:

Example 1:

Should the passability, caused by migration barriers, of a lower mountain stream be recreated considering its morphological structure and semi-naturalness? It also deals with a consistently severely acidified stream (the pH value is generally under 5.5 all year round) which, due to its chemical water condition, leads to unfavourable living conditions for fish and macrobenthic growth. It transpires that it is not sensible to carryout passability measures in this stream at the moment.

Example 2:

A lower mountain stream could be made passable from its source to its mouth by the transformation of two travel barriers. Intensively managed fish ponds lie along the stream in a neighbouring channel. In a nearby stream, passability between the source and stream mouth could be created by removing or modifying five travel barriers. The ultimate outcome was that it was preferable to remove or modify the five travel barriers. The organic water quality in the stream with the two obstacles, because of the input of severely impacted organic water from the fishponds, was generally worse than quality category 2. No sustained improvement in water quality is expected in the foreseeable future, therefore no passability measures will be planned here in the short term.


  • SCHABER-SCHOOR, G. (2007): Kleine Gewässerläufe im Wald – Grundlagen für den Erhalt und die Entwicklung naturnaher Bachläufe in bewirtschafteten Wäldern. Schriftenreihe Institut für Landepflege, Culterra 49, Freiburg, 247 S. u. Anhang


  • This article is part of the "Handbook Forest and Water"