Background

Many river systems worldwide have been impacted by dams to fulfill human water needs and for protection against floods (Dynesius and Nilsson, 1994; Nilsson et al., 2005). Large reservoir dams can serve several purposes. They can attenuate floods by cutting of the peak flows, secure water supply and navigation and they can be used to generate hydropower (Poff and Hart 2002).

According to the Serial Discontinuity Concept developed by Ward and Stanford (1983), dams are viewed as discontinuities within the river continuum. The concept predicts changes in various physical and chemical parameters like substrate size and light at the bottom as a result of damming. The concept is later refined to include floodplain interactions (Ward and Stanford, 1995) and has been tested in different river systems (Stanford and Ward, 2001).

Dams can negatively impact the functioning of river ecosystems by changing ecological, chemical as well as physical system properties. The magnitude of these effects depends on the type of system, the dam size and the reservoir operations (Figure 0.1, Poff and Hart 2002).

Figure 0.1. Flow chart illustrating how dam operation and size affects biophysical processes in rivers (source: Poff & Hart, 2002).

This summary focusses on three major pressures that affect ecological functioning of river systems: 1) altered flow regime, 2) reduced sediment transport, and 3) biotic fragmentation.

Altered flow regime

Natural flow variations are necessary to support different life events of aquatic organisms. For instance, flood peaks provide migration cues for fish. High flows create a connection between channel and floodplains and shape physical characteristics of the river channel, while base flow is necessary to maintain suitable habitats for aquatic organisms (Bunn and Arthington, 2002).

Dams affect the magnitude, timing, duration and frequency of high and low flows (Magilligana et al., 2005). Based on an extensive review, Poff and Zimmerman (2010) found that 92% of the reviewed studies show a negative ecological response to various types of flow alteration. Also, larger changes in flow generally have greater risks of ecological change.

A reduction of frequency and extent of floodplain inundation can lead to reduced connectivity between channel and floodplain and can change the vegetation successional trajectories (Poff et al., 2010). When flows are reduced, vegetation can encroach the channel, leading to channel narrowing  and  reduced conveyance capacity  which in turn can lead to increased flood risks in case of a high flow event (Williams and Wolman, 1984). In general, flow stabilization leads to a decline of the riparian vegetation belt and creates a gradual replacement of wetlands by terrestrial species (Van Oorschot et al., 2018).

Reduced sediment transport

Reservoirs can become sediment traps. As a result, the total sediment load downstream of the dam is reduced (Kondolf, 1997). Sediments are necessary to build physical features in the river like banks and islands that serve as habitats for many species. Reduced sediments can lead to erosion of downstream areas and bed armoring, which is most pronounced directly downstream of the dams. This leads to entrenched river stretches that loose channel-floodplain connections that are important for many species to complete their life cycles. However, it is clear that there is a large variability between river reaches in physical and ecological magnitude and rates of change (Petts and Gurnell, 2005). 

Additionally, sediments are deposited downstream and help to shape delta areas. Sediment retention by dams is linked to accelerated subsidence of deltas, making delta areas even more susceptible to flooding by sea level rise (Anthony et al., 2015).

Since coarser sediments that travel in bed load behave differently than silt and clay that are in suspension, a distinction has to be made between the two types. Coarse sediments are almost always captured in reservoirs and reduced supply of coarse sediments downstream can result in channel incision and degradation of aquatic habitats, e.g. spawning areas for fish (Kondolf et al., 2014). Fine-grained sediments like silt and clay carry adsorbed organic matter and particulate organic matter that are important food sources for the downstream ecosystem (Piman and Shrestra, 2017) and they help to build estuarine mud flats. Animals that feed on organic matter form the basis of the food web and many higher trophic levels depend on these. So reduction of these sediments will affect the nutrient composition in the river system and thereby the whole food web. This wash load can pass smaller dams, but when retention times increase, also fine-grained sediments are captured in the reservoir (Kondolf et al., 2014).

Sediment retention is an undesirable side effect of dams that also creates problems for reservoir managers. Increased sedimentation decreases the life span of the dams and reduces reservoir capacity. Therefore, many reservoirs are periodically flushed. However these flushing events can also negatively affect aquatic organisms. For instance, Crosa et al., (2010) find decreased fish densities and biomass and a drastic reduction in zoobenthic assemblages after large flushing events.

Figure 0.2. Sediment trapped in river corridor (Source: Baran et al.,2015) 

Biotic fragmentation

Dams are physical obstructions in the river and therefore disconnect fish migration routes. Fish migrate for different reasons, for instance short distances in search of food or from main channel to floodplain or longer distances for spawning. Especially long distant migratory fish species, i.e. anadromous fish species that spawn in freshwater and grow up in salt water (e.g. salmon) and catadromous fish species that spawn in salt water and grow up in freshwater (e.g. eel) are negatively affected by disturbed longitudinal connectivity (Puijenbroek et al., 2019). Many fish die or are mutilated while trying to pass the turbines of hydropower dams (Deng et al., 2005).

To support fish migration, many types of fish traps have been designed. An effective fish trap has to tick many boxes, e.g. attraction flow, location of the entrance and suitable hydraulic conditions.  Each fish has different characteristics and preferences, which makes it nearly impossible to design a fish trap that fits the needs of all migratory fish in the river. So a dam inevitably leads to a reduction of migratory fish (Puijenbroek et al., 2019). Robust fish passage monitoring programs are lacking, which is critical to assess the effectivity and disseminate knowledge to be used in other river systems (Silva et al., 2018)

Besides being a physical barrier, reservoirs create changes in habitats. Reservoirs are stagnant, deep water bodies and that differ from the free-flowing rivers that contain more dynamic properties like pool-riffle sequences, variable flow conditions between inner and outer banks and bar and island formation. Different species prefer different conditions. Rheophilic species prefer faster flowing water, while limnophilic species prefer more stagnant and vegetated water. In between are eurytopic species which are considered generalists and most tolerant to either degradation of hydromorphological conditions or water quality. When a river becomes a succession of reservoirs, species assemblages will change from rheophilic to eurytopic species and the habitat of rheophilic species will be drastically reduced and fragmented.

Sustainable dam operation

Approximately 50% of the countries in the world are members of the International Commission On Large Dams (ICOLD). This non-governmental organization provides a platform for exchange of knowledge in dam engineering. Its goal is to ensure dams are built safely, efficiently, economically and without detrimental effects on the environment[1] .

New research on methods to better manage the flow and sediment release and the design of fish ladders provide valuable insights to limit ecological consequences of dams.

Managing flows

The concept of environmental flows has been developed to balance interests in river basin management. Environmental flows describe the quantity, frequency, timing and quality of water and sediment flows necessary to sustain freshwater and estuarine ecosystems and the human livelihoods and well-being that depend on these ecosystems (Worldbank 2018, amended from the Brisbane declaration 2007). An environmental flow assessment investigates how much the current flow regime has been altered from the natural flow regime, how changes in flow regime affect downstream ecosystems and how all benefits and impacts are distributed amongst various stakeholders. A definition of an environmental flow can help to create a flow regime that balances both ecological and socio-economic interests and should preferably embrace both water and sediment.

In the Murray-Darling basin in Australia there is even an authority responsible for the delivery of environmental flows. This authority determines the sustainable limits for water diversion and prescribes the balance between consumptive and environmental uses of water (Banks and Docker, 2014). Depending on the amount of water that is available, choices are made to release water to inundate parts of the downstream wetlands to facilitate growth of diverse vegetation types. For instance, when small amounts are available, they are used to support reed beds, and in case of larger amounts of water, larger wetland areas are connected to the main channel. Extensive vegetation monitoring related to climatic conditions and inundation schemes are used to better determine the water needs for these wetlands (Dyer et al., 2018).

Dam removal

More drastic than applying environmental flows would be the complete removal of a dam. This can be considered when its functionality has become needless. Dam removal recently gets more appreciation and momentum, because of its higher environmental effectiveness compared to fish migration facilities and environmental flows [2]^,[3].

References

Anthony, E.J. et al. (2015). Linking rapid erosion of the Mekong River delta to human activities. Nature, 5.

S.A. Banks & B.B. Docker (2014) Delivering environmental flows in the MurrayDarling Basin (Australia)—legal and governance aspects, Hydrological Sciences Journal, 59:3-4, 688-699, DOI: 10.1080/02626667.2013.825723

Bunn, S.E., Arthington, A.H. (2002).Basic principles and ecological consequences of altered flow regimes for aquatic biodiversity. Environmental Management, 30(4), 492–507.

Crosa, G., Castelli, E., Gentili, G. et al. Effects of suspended sediments from reservoir flushing on fish and macroinvertebrates in an alpine stream (2010). Aquatic Sciences 72:85. https://doi.org/10.1007/s00027-009-0117-z.

Deng, Z., Guensch, G.R., McKinstry, C.A., Mueller, R.P.; Dauble, D.D.; Richmond, M.C. Evaluation of fish-injury mechanisms during exposure to turbulent shear flow. Can. J. Fisheries Aquat. Sci. 2005, 62, 1513–1522. 

Dyer, F. et al. (2018). Wetland and vegetation responses to environmental water and flooding: the lower Lachlan river system, Australia. Abstract Ecohydraulics Conference, Japan.

Dynesius,M.,Nilsson,C. (1994).Fragmentation and flow regulation of river systems in the Northern third of the world. Science, 266(5186), 753–762.

Kondolf, M.G. (1997). Hungry Water: Effects of Dams and Gravel Mining on River Channels. Environmental Management, 21(4), 533-551.

Kondolf, G. M. et al. (2014), Sustainable sediment management in reservoirs and regulated rivers: Experiences from five continents.  Earth’s Future, 2, 256–280. doi:10.1002/2013EF000184.

Nilsson, C., Reidy, C., Dynesius, M., Revenga, C. (2005).Fragmentation and flow regulation of the world’s large river systems. Science, 308(5720), 405–408.

Magilligana, F. J., Nislow, K.H. (2005). Changes in hydrologic regime by dams. Geomorphology ,71, 61–78.

Petts, G.E., Gurnell, A.M. (2005).Dams and geomorphology: research progress and future directions. Geomorphology 71, 27–47.

Poff, N. L., Richter, B. D., Arthington, A. H., Bunn, S. E., Naiman, R. J., Kendy, E., … Warner, A. (2010). The ecological limits of hydrologic alteration (ELOHA): a new framework for developing regional environmental flow standards. Freshwater Biology, 55(1), 147–170. doi:10.1111/j.1365-2427.2009.02204.x

Poff, N. L., & Zimmerman, J. K. H. (2010). Ecological responses to altered flow regimes: A literature review to inform the science and management of environmental flows. Freshwater Biology, 55, 194–205. doi:10.1111/j.1365-2427.2009.02272.x

Piman, T. and Shrestha, M. (2017). Case Study on Sediment in the Mekong River Basin: Current State and Future Trends. UNESCO and Stockholm Environment Institute (SEI).

Future Trends. UNESCO and Stockholm Environment Institute (SEI) Poff, L.N., Hart, D.D. (2002). How Dams Vary and Why It Matters for the Emerging Science of Dam Removal. BioScience, 52(8) 659-668

Puijenbroek, P.J.T.M. van, A.D. Buijse, M.H.S. Kraak, P.F.M. Verdonschot. (2019). Species and river specific effects of river fragmentation on European anadromous fish species. River Research and Application, 35, 68-77.  http://dx.doi.org/10.1002/rra.3386.

Van Oorschot, M., Kleinhans, M., Buijse, T., Geerling, G., Middelkoop, H. Combined effects of climate change and dam construction on riverine ecosystems. Ecological Engineering, 120, 329-344.

Ward, J. V., & Stanford, J. A. (1983). The serial discontinuity concept of lotic ecosystems. Dynamics of lotic ecosystems, 10, 29-42.

Ward, J. V., & Stanford, J. A. (1995). The serial discontinuity concept: extending the model to floodplain rivers. Regulated Rivers: Research & Management, 10, 159-168.

Worldbank (2018). Environmental Flows for Hydropower Projects.  Good practice handbook , Guidance for the Private Sector in Emerging Markets.

Silva et al. (2018). The future of fish passage science, engineering, and practice. Fish and Fisheries. 19, 340–362. DOI: 10.1111/faf.12258

Stanford, J. A., & Ward, J. V. (2001). Revisiting the serial discontinuity concept. Regulated Rivers: Research & Management, 17, 303-310.

Williams, G.P., Wolman, M.G. (1984). Downstream effects of dams on alluvial rivers. Tech. rep. United states department of the interior, Washington,D.C.

[1] https://www.icold-cigb.org/

[2] https://www.damremoval.eu/

[3] https://www.americanrivers.org/conservation-resource/american-rivers-dam-removal-database-now-available-public/