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A natural experiment reveals the impact of hydroelectric dams on the estuaries of tropical rivers

A natural experiment reveals the impact of hydroelectric dams on the estuaries of tropical rivers

INTRODUCTION

During the last decades, many voices have expressed concern about the environmental impact of large dams. Although hydroelectric power is renewable and can reduce CO2 emissions derived from the burning of fossil fuels for the generation of thermoelectricity, large hydroelectric dams may also have important adverse consequences on the environment because they alter river hydrology, nutrients concentration and amounts, and the lifecycles of species that depend on freshwater habitats.

Reductions in water pulses downstream can increase substrate salinity, lower the groundwater table, and make water unusable for drinking and irrigation (1). Decomposition of organic matter drowned in the dam’s reservoir can emit large amounts of methane and can also promote the leaching of toxic metals from the flooded minerals and rocks in the reservoir (1).

Sediments that are crucial for natural cycles are trapped in the dam’s reservoir affecting the normal functioning of downstream ecosystems, including wetlands and coastal lagoons (1). Thirty-three of the largest deltas in the world are now rapidly receding and sinking, with serious consequences for regional agriculture and livelihoods [see (2) and references therein].

Although alterations of the water flow by damming and diversions are not the only cause of deltaic degradation (aquifer water extraction and oil drilling, for example, seem to be playing a role too), they are the most important common cause globally.

While in many developed nations strong public opinion has gradually developed against dams and the era of big dam building is considered to be over [e.

g., (3)], many developing countries still see the construction of dams as a strong incentive for development, capable of mobilizing investment, activating the demand for industrial supplies such as steel and concrete, offering employment, and providing renewable energy.

However, the environmental costs in terms of loss of land, alterations to the water regime, habitat disruption for riverine species, sediment trapping in the reservoir, and the degradation of water quality are often not weighed appropriately against the purported benefits. Strong debates persist around this issue throughout the developing world, and many studies [e.

g., (4–7)] have tried to tackle the complex issue of economic tradeoffs between hydropower generation and the associated environmental impacts in an attempt to achieve a goal of generating electricity while at the same time avoiding many of the environmental costs of large dams.

Tropical rivers normally carry large amounts of suspended sediments that feed sandbars, deltas, and accretional coastlines. Most of these sediments become trapped in the body of large reservoirs, changing the coastal dynamics in the estuaries of dammed rivers.

The impact of large dams in coastlines, estuaries, deltas, and lagoons is commonly understudied in the environmental studies of hydroelectric projects. Furthermore, when evaluated, it has historically been approached by analyzing time series documenting the state of the estuary and the lower basin before and after the damming of the river.

This approach, however, is based on repeated observations along the same river and not based on true statistical replicates. Long-term change can be the result of many other factors that have varied with time and not necessarily of the dam itself.

A comparative study between dammed and undammed rivers may provide critically needed evidence.

The Pacific coast of Mexico in the States of Nayarit and Sinaloa provides an ideal setting for this approach.

Four rivers run roughly parallel to each other down the Sierra Madre from the Mexican Plateau and reach the Pacific coast relatively near to each other. Two of them have been dammed for hydroelectricity, while the other two still flow free into the coastal estuaries.

The southernmost one, the Santiago River, harbors four dams (Aguamilpa, finished in 1994; El Cajón, 2007; La Yesca, 2012; and Santa Rosa, 1964), and the northernmost one, the Fuerte River, was dammed in 1956 by the El Mahone dam. The remaining two rivers, the San Pedro and the Acaponeta, still flow free onto the coastal plains (they both have some small impoundments for irrigation but are still largely running free).

All four rivers flow into large coastal lagoon systems: Marismas Nacionales (Mexico’s largest coastal wetland) and the Ahome wetlands.

Using these rivers as a “natural experiment,” we analyzed the way sediment trapping by dams affects the estuaries of tropical rivers.

If the sediment reduction imposed by the operation of the dams distinctly affects the lagoon system, then significant differences in coastal dynamics should be evident when comparing the estuary of the two undammed rivers against the estuaries of the Santiago and the Fuerte rivers, which have been dammed for 23 and 61 years, respectively.

Thus, the objective of our study was to analyze the geomorphologic dynamics of the coast immediately adjacent to the estuaries of two dammed rivers and compare it with that of the region’s two nondammed rivers, describing the differences between the two systems.

We also calculated the impact of sediment reduction in terms of the loss of ecosystem services and emissions of carbon into the atmosphere. Last, we present a balance between the benefits and costs of damming a tropical river for energy production against the alternative of letting the river flow free into the ocean.

The study area

All four rivers descend from the Mexican highlands across the Sierra Madre into the wetlands of the Pacific coast of Mexico. Three of them, Santiago, San Pedro, and Acaponeta, reach the coast at Marismas Nacionales, the largest (ca.

1800 km2) tropical lagoon complex in the Pacific coasts of the American continent. The fourth and northernmost river, Río Fuerte, reaches the coast in the more northern Ahome wetlands in the Gulf of California (table S1).

The Fuerte River was dammed for hydroelectricity in 1956 by the El Mahone dam and the Santiago River in 1994 by the Aguamilpa dam. Other dams have been built since in both rivers upstream of these lower-basin reservoirs.

Currently, only 4% of the area of the Fuerte and 2% of the Santiago watersh are free of dams and can produce unobstructed runoff feeding the lower basin and reaching the ocean at Punta Ahome (25°57′09″N, 109°26′37″W) and Boca del Asadero (21°38′05″N, 105°26′44″W), respectively. Most (95%) of the flow in these two rivers goes through reservoirs where a large part of their sediments are trapped.

The other two rivers, Acaponeta and San Pedro, do not have dams in their lower basin and are largely “free” rivers. Both come down across the Sierra Madre from the highlands of Durango; the Acaponeta reaches the Agua Brava Lagoon in the heart of the Marismas Nacionales wetlands from where it drains into the Pacific Ocean through an estuary known as Boca de Teacapán (22°32′05″N, 105°45′15″W).

The San Pedro reaches the wetlands at the Mezcaltitán lagoon from where it drains into the ocean at Boca de Camichín (21°43′58″N, 105°29′32′′W). The Acaponeta has no dams along its course, although some of its water is diverted, mostly during the dry season, for local irrigation.

The San Pedro has two dams in its upper watershed, which are used for irrigation in the highlands of Durango, but most (ca. 75%) of its watershed drains unobstructed into the sea.

Detailed information on the four rivers is provided as Supplementary Materials.

Marismas Nacionales.

Marismas Nacionales (National Marshlands) is a large, complex lagoon system that runs parallel to the coast for some 150 km in the States of Nayarit and Sinaloa from the historic port of San Blas in the south to the village of Escuinapa in the north. One of its most distinctive traits is the presence of two well-defined geomorphologic units: A system of large inner lagoons surrounded by mangrove forests and a coastal system of parallel beach ridges that forms a spectacular succession of accretion lines separating the lagoons from the coast of the Pacific Ocean (Fig.

1). The coastal ridges of Marismas Nacionales have been studied in great detail since the 1960s by Curray et al.

(8–10). The ridges are formed by coastal accretion derived from the continental input of sediments brought down from the Sierra Madre into the coastal plains by three main rivers: Acaponeta, San Pedro, and Santiago.

After post-Pleistocene sea-level rise stabilized, ca. 5000 years before the present, the ridges started to form at a mean rate of approximately 1 m every 12 years.

The ridges are made up of a mixture of sediments derived from the longshore transportation of the Santiago and San Pedro rivers and the onshore transportation of reworked sediment from the now drowned paleo-river delta constructed on the continental shelf during the previous sea-level low stand in the Pleistocene and early Holocene (11). The current boundary between the large inner lagoons and the ridge system marks the ancient coastline from where the ridges grew.

Curray’s seminal studies allow calculating the rate at which the Marismas coastline expanded and grew into the ocean shelf, as well as the amount of sediment brought in the past by the rivers that gave origin to this unique system. The dynamics of sediment deposition along the coastline, and particularly of the coastal ridge systems, is at the core of current concerns about the environmental impact of hydroelectric dams.

Reductions in sediment loads from sediment trapping in reservoirs may reverse the historic accretion of the lagoon systems, opening the way to the erosion of sandbars and coastal ridges and, potentially, to the destruction of the coastal wetland ecosystems.

Fig.

1 The Marismas Nacionales lagoon system.

(A) Satellite image of Marismas Nacionales showing the inland system of Pleistocene and early Holocene lagoons and the coastal system of late Holocene beach ridges separating the lagoons from the sea (photo credit: Google Earth).

(B) Aerial view of the parallel beach ridges derived from successive events of coastal accretion (photo credit: J. Rojo, iLCP).

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