1. INTRODUCTION
2. RELATED TOPICS AND DELTAFACTS
3. STRATEGIC CONTEXT
4. GRAPHICAL ABSTRACT
5. CONTENT
6. COSTS AND BENEFITS
7. SPECIFIC CONDITIONS
8. KNOWLEDGE GAPS

1. Introduction

It is essential to assess the effectiveness of restoration measures and to determine of environmental or ecological targets in stream and river restoration projects are achieved. This allows fine-tuning or adapting restoration measures, and applying insights from previous projects in future projects. Furthermore, availability of monitoring results facilitates communication with internal and external stakeholders. Also clearly explaining why certain measures are taken, and what their expected impacts are is important for gaining support for implementation. However, despite its relevance, monitoring of stream and river restoration projects is often lacking (Dos Reis Oliveira et al., 2020, https://doi.org/10.1016/j.jenvman.2020.110417). This gap is commonly explained by insufficient availability of resources and a lack of familiarity with setting up monitoring programs.

Effective project-based learning in restoration initiatives requires documentation of the restoration process steps and the subsequent environmental and ecological changes in the river ecosystem. The purpose of this Deltafact is to facilitate this process. It helps water managers involved in stream and river restoration in choosing the appropriate monitoring design and biological parameters to assess their projects' ecological effectiveness. Our recommendations aim to optimize future monitoring designs, establishing causal relationships between restoration measures and biological outcomes.

2.  Related topics and Deltafacts

Deltafacts

• Ecological monitoring [https://www.stowa.nl/sites/default/files/assets/DELTAFACTS/Deltafacts%20NL%20Waterkwaliteit%20PDF/Monitoren%20van%20de%20ecologie-converted.pdf]
• Strategies for assessing the effectiveness of stream restoration projects [https://www.stowa.nl/sites/default/files/assets/DELTAFACTS/DF%20Lumbricus/DF%20PDF/Monitoring%20effectiviteit%20beekherstelprojecten.pdf]  

COSAR Deltafacts

• Factors contributing to successful river restoration
• Legacy effects affect river restoration outcomes
• Use of social media data in river restoration
• DAPSIR: a predictive model of river restoration outcomes
     

3. Strategic context 

Since the establishment of the Water Framework Directive (WFD) there has been a considerable effort to standardize biological monitoring of European freshwaters. This effort has resulted in guidelines for measuring a specified set of parameters using standard methods at fixed measuring points at certain frequencies (Hering et al., 2004,  https://doi.org/10.1023/B:HYDR.0000025255.70009.a5). This systematic approach has been widely adopted among EU member states and involves the integral measurement of various biological, hydromorphological, and physico-chemical parameters at designated sites.

While this method establishes a baseline understanding of a river's ecological water quality, it often fails to capture the dynamic and complex nature of aquatic ecosystems. To determine the effectiveness of restoration measures, a more targeted monitoring approach is needed that accounts for other environmental factors influencing the ecosystem and goes beyond generic water quality assessments. Ultimately, this approach should result in a more refined, scientifically robust monitoring approach that better captures the effects of restoration efforts and supports effective water management decisions.

4. Graphical abstract

Not applicable.

5. Content

Which monitoring should be implemented to collect the necessary information?

Different monitoring designs can be used to determine the effects of stream and river restoration measures on ecological and physical-chemical parameters, among others (Figure 1). The extent to which the results can be associated with the restoration measure (i.e., the causal relationship) and the statistical reliability depend on the chosen design.

Before/After-Control/Impact

The most reliable design for determining the causal effects of restoration is the before/after - control/impact (BACI) design (Stewart-Oaten et al., 1986; https://doi.org/10.2307/1939815). Measurements are carried out multiple times before and after the measure is applied in the section of the stream or river to be restored, as well as in one or more control sections. A control section has characteristics similar to those of the section to be restored, e.g., in terms of hydromorphological conditions and species assemblages. However, finding suitable control sections is often challenging because of the environmental heterogeneity present in most river systems. In practice, upstream sections close to the section to be restored (but not impacted by the applied measures) are often the most suitable option.

The BACI design corrects for the initial state (e.g., community composition before restoration) in the section where the measures are implemented, as well as for changes unrelated to the measures (e.g., weather-related flow differences) that occur over time in the control section. To acquire reliable results, it is advised to measure at least three times before implementing the restoration measures and at least four times after applying them.
 

Because ecosystems may respond slowly after restoration, it is advisable to measure over a longer time period (Smokorowski & Randall, 2017, https://doi.org/10.1139/facets-2016-0058). For water managers, this means that, to apply this design effectively, they must start the monitoring program well before implementing the measures and plan for a long monitoring period afterwards. For example, measurements should take place in the years -3, -2, -1, 1, 2, 5, and 10.

Before/After

In some cases, control sections are unavailable. In such situations, the before/after (BA) design could be a solution, with measurements taken before and after the restoration implementation. The reliability of a time series analysis depends on the number of measurements carried out over time. In this case, it is crucial to have a sufficient number of measurements before implementation to capture potential variations (e.g., in community composition) in the system and determine the effect of the restoration measure.
However, this design is vulnerable to detecting changes unrelated to the measure because all changes in the studied parameter after restoration are attributed to the measure. Unlike the BACI design, no correction by the control can be carried out. Therefore, it is recommended to sample multiple locations within a catchment area using the multiple before-after (mBA) design, which enables you to account for spatial and temporal differences resulting from changing environmental conditions over time.

Control/Impact

Finally, the effectiveness of the measures can be determined by comparing sites where the measures have been implemented with sites that have the same characteristics and are unrestored. This is the multiple control-impact (mCI) design. This design corrects for effects unrelated to the measures by cross-comparing a series of restored-unrestored site pairs. However, this makes the design vulnerable to small sample sizes. Furthermore, it does not provide insight into the development of the effects of the measures or changes in assemblage composition over time. This limitation can be overcome by selecting sites that vary in the time since implementation of the measure (the so-called "place-for-time substitution") or the extent of the intervention. This allows one to determine correlative relationships. One advantage of this design is that it can be used after the measure has been implemented. With multiple restored-unrestored pairs for a certain measure, insight can be gained into the generic effects of the measure.
 

Figure 1: Monitoring designs that can be used to determine the effects of restoration measures (Verdonschot et al. 2020, The change over time of parameter x is measured before (B) and after (A) the application of the restoration measure. The BACI design contains a restored impact location (I) and an unrestored control location (C), where measurements are carried out before and after the measure is implemented. In the BACI design, measurements are taken only at the impact location. In the CI design, measurements at the impact and control locations are compared. In a CI design, to determine the effect of parameter x, multiple impact locations (mCI) that vary in factor y can be compared, which allows you to correlate the effect to the observed change.

How can the effects of restoration be determined based on the collected data?

After field sampling and processing, there are several ways of analysing the acquired data. Here were present two widely applicable approaches: assessing effects based on temporal response curves and using effect sizes to determine the magnitude of the change according to the pre-restoration state.

Temporal response curves

One option is to compare the temporal response curves of a restored site and its unrestored control for a metric of interest, for example, comparing species assemblages of restored and control river sections. This approach provides information about the extent to which a restored site diverges from the pre-restoration state and whether it reaches a specific reference or target state (the restoration goal) over time (Figure 2). An example could be found in Verdonschot et al. (2015) https://doi.org/10.1111/fwb.12479. However, such a spatio-temporal comparison can be limited methodologically, as it requires a BACI design, as well as a long time series of data. Furthermore, it is limited in its ability to provide generalized patterns across a large number of sites.