Rivers

Rivers are defined as lotic biomes (from the Latin lotus, meaning “washed”); that is, inland watercourses that unidirectionally flow through topographic gradients in the landscape. The English word comes from the Vulgar Latin riparia (“riverbank, seashore, or river”). This term is frequently utilized to indicate relatively large water streams. The structural and functional characteristics of rivers are best understood at the watershed level. A watershed (or drainage basin, or catchment) is a topographic depression drained by a hydrogeographic network of tributary channels, which coalesce into a main river channel. Within the watershed, surface water flows from the area of maximum potential energy of the depression (the topographic divide; or below the ground level, the phreatic divide) to the point of minimum energy, which usually is another water body (e.g., a lake or sea).

Water influx entering the watershed as ice melt, snow, or rain (precipitation) is either intercepted by the vegetation, absorbed, transpired, or evaporated at the plants' surface; or by the substrate, where it can infiltrate the soil in a compartment where water moves by capillarity (the vadose zone), which is renewed by evapotranspiration. Below the vadose zone, groundwater is collected in a water-saturated compartment superiorly delimitated by a plane called the water table: the phreatic zone. Water exceeding the soil infiltration capacity (which in turn depends on the storage capacity of the vadose zone) can move in the gravitational field as overland flow; similarly, lateral subsurface flows can occur within the vadose zone, and base flows within the phreatic zone, which can also surface as springs. Hydrological connections are present among all these compartments. Therefore, the hydrology, chemistry, and biology of a river largely depend on the climate, geology, and vegetation coverage of the watershed and its groundwater connections.

Global Patterns

River and stream channels cover about 0.1 percent of land surfaces and contain only approximately 0.0001 percent of the total volume of water on Earth (about 1/100th of that contained in lakes), or approximately 288 cubic miles (approximately 1,200 cubic kilometers). Nonetheless, the total drainage area of the 30 major river basins on Earth (about 20 systems, mostly longer than 1,243 miles, or 2,000 kilometers) covers about 25 percent of the total land area of all continents, or 14.4 million square miles (37.2 million square kilometers), 1.9 million square miles (5 million square kilometers) by the Amazon River Basin alone. Approximately 23,991 cubic miles (100,000 cubic kilometers) of water precipitates on continental masses each year: 65 percent of this amount returns to the atmosphere by evapotranspiration, and 35 percent reaches the oceans as stream and base flows as discharge. Rivers erode large amounts of dissolved and particulate materials collected by runoff within their watersheds, transporting them from the land to the sea. Such dynamic nature is exemplified by their water residence time, which is on average much smaller (throughflows, or renewal rates: 12 to 20 days) than in other aquatic systems, such as freshwater and saline lakes (1 to 1,000 years) or oceans (3,100 years). At the landscape level, hydrological regions where rivers originate and reach the sea are called exorheic; those where rivers originate and do not reach the sea are called endorheic; and those where rivers do not originate (e.g., desert regions) are arheic. Changes of hydrological regimes can occur during regional or global climatic changes.

Geomorphology of River Channels

The river channel is a through-shaped geomorphic entity defined by soil or rock boundaries (banks and bottom). The river flow, whose velocity is determined by the balance of gravitational and frictional forces, erodes such solid boundaries, transporting materials on the bottom (bedload) or in the water column (suspended load), and dissipating energy. For this reason, the maximum flow velocity is in the middle of the channel, rapidly decreasing toward the water–air and water–substrate interfaces (boundary layers). This flow also interacts with the lateral and vertical control determined by the topography (valley), which determines whether the river is either relatively free to move laterally (unconstrained), thereby forming meanders and flood plains; or not free to move laterally (constrained), such as in incised or entrenched valleys, where scarce or absent riverine vegetation and no flood plains occur. The topographic gradient and other factors, such as the extent and type of sediment supply, then determine the channel morphology (e.g., straight, meandering, braided, or anostomosing).

The degree of water penetration in the interstitial space of the bottom and bank sediments of river channels is determined by the topographic gradient, sediment composition, and flow characteristics. Below the bottom, there is a soil layer where water actively infiltrates by advection in the hyporheic zone, an inch (few centimeters) to three feet (one meter) deep; further below, there is a water-saturated layer where groundwater prevails (the interactive hyporheic zone). The boundary between these two zones changes spatially and temporally because of several factors, such as the penetration of animals and plants (bioturbation), and floods.

In general, base flow inputs into the river from the phreatic zone dominate upstream (e.g., in headwaters); while downstream, as the topographic gradient is reduced, subsurface flows from the river water into the streambed become more important (e.g., in flood plains and deltas); in these latter conditions, near to the minimum energy level, groundwater inputs to the stream flow are minimal, and hydrological boundaries between river and groundwater extend laterally. Geomorphological changes occur along the river as the kinetic energy decreases and the sedimentary budget becomes positive, such as when it is rapidly reduced by flowing into a larger body of water forming a delta; or after a sudden transition from a narrow valley to a plain, forming a fluvial fan. Flow patterns also undergo considerable temporal changes. Periodic or irregular temporal changes of the stream flow (e.g., floods) are related to climatic events (e.g., storms), drainage area, and type of interactions between overland and base flows.

Temperature

Apart from water velocity and turbulence, one of the most important physical factors affecting the composition of fluvial biotic communities and metabolism is temperature. Rates of temperature change in rivers are mainly determined by solar and atmospheric radiation; the back radiation released into the atmosphere at the water surface; and changes of volume. Hydrology, climate, and insulation thus interact and determine rivers' thermal regime. River ecological zonations are heavily affected by downstream changes in temperature along the elevational gradient, and biotic fluctuations are affected by the cumulative number of days over a year when the temperature is above 32 degrees F (0 degrees C). The energy provided by turbulent conditions makes ice formation rarely complete in running waters, especially at riffles; small ice crystals can form in turbulent and slightly supercooled conditions in the water column (frazil or slush ice) or on substrates (anchor ice), affecting both the sedimentary balance (erosion of bottom and banks) and the life of benthic organisms.

Metabolism and Nutrients

The metabolism of fluvial ecosystems is mainly affected by the spatial and temporal budgets of a few chemical compounds (oxygen, inorganic carbon, nitrogen compounds, and phosphorous compounds), the abundance of which is determined by both biotic and abiotic processes. In streams of lower order, with colder and more turbulent waters, oxygen is often approximately at saturation, declining during periods of higher temperature (e.g., summer in temperate regions), as equilibrium solubility decreases. However, oxygen levels can also markedly fluctuate because of chemical and biological processes, especially in larger, well-illuminated, and relatively sluggish rivers, where bacterial respiration and photosynthetic activity by plants and phytoplankton are more intense. Such changes are mainly associated with spatial and temporal changes of inputs and outputs of organic matter, which rapidly reacts with oxygen in chemical or biochemical processes. Examples are anoxic groundwater inputs, periodic floods, or inputs of leached materials from leaf fall from adjacent terrestrial systems. Hypoxic or anoxic conditions are often found in microenvironments within microbial communities attached to solid substrates, and in the interstitial water of saturated sediments.

Respiration and decomposition dominate over primary production within rivers; furthermore, surface and groundwater inputs from runoff are often both anoxic and hypercarbic. These effects are more evident in smaller rivers of lower stream order. As a result, variations in carbon dioxide (CO₂) concentration and pH are common. Lower pH and lower alkalinity levels can occur during periods of reduced flow or in areas with slower flows, and in correspondence with loadings of dissolved and particulate organic matter. In particular, softwater rivers with low alkalinity (e.g., flowing on granitic or sandstone sediments) can be rapidly acidified by modest inputs of acidic water from atmospheric sources or runoff. On the other hand, acidic inputs in hardwater rivers (e.g., flowing on carbonatic rocks) may increase alkalinity and equilibrium pH because of increased rates of carbonate weathering in the watershed.

The concentration and net downstream transport of dissolved nutrients depend on their rates of uptake and release between the streambed and the water column along the river, in a process called nutrient spiraling. The retentiveness of nutrients depends on the extent by which their downstream transport is delayed by physical, chemical, and biological processes (mainly from microbial action), relative to the transport of water. Nutrient limitations of primary production and of heterotrophic microbes' production are associated with faster cycling rates and higher biological demands of a particular nutrient (e.g., nitrates or dissolved phosphorous).

Lower stream order rivers are often oligotrophic, both because of the smaller size of their drainage basins and the lower anthropogenic impact, which limit nutrient loadings. Nonetheless, nutrient limitations are less common in these latter systems, since most biota are attached to the substrate, and recycling within the community is more efficient than in larger and higher stream order rivers, where nutrient retention is reduced.

Dissolved inorganic and organic phosphorous is more controlled by biological uptake than by discharge, while the opposite is true for particulate phosphorous, which is more slowly metabolized. On the other hand, accumulated particulate phosphorous can be released downstream in large pulses, during precipitation-mediated events of higher discharge.

System Ecology and Productivity

At the ecosystem level, terrestrial and aquatic systems are connected within watersheds by water movements, and downstream fluvial sections are unidirectionally influenced by upstream sections. Especially in streams of lower order, the majority of the organic matter is allochthonous, deriving from adjacent floodplains, wetlands, and terrestrial systems. Spatial and temporal changes of flow patterns are accompanied by changes of the associated biotic communities, affected by the different types of habitats that flowing patterns create; in particular, stream biota are generally adapted to flowing waters. For the rest, spatial and temporal variations characterize flow, chemical and biological conditions and generalizations are rather difficult. Nonetheless, general models were proposed (e.g., the river continuum concept, or RCC) to describe how spatial and temporal changes of the functional composition of fluvial communities, organic matter supply, and resource partitioning can be related to stream order, current velocity, and intensity of biological activity.

Within the water column in the river channel, photosynthetic activity (e.g., phytoplankton) is inhibited by turbulence and turbidity induced by water flow and shading from the riparian vegetation. The productivity of submersed macrophytes is generally only slightly higher than that of phytoplankton, with higher contributions from attached algae and biofilms in shallow and less turbulent lotic habitats. Also, the biomass of zooplankton is generally negatively correlated with flow velocity, and planktonic animals compensate for the effect of flow and downstream transport with reduced life cycles, benthic lifestyles, and colonization of littoral backwater habitats. The bacterioplankton heavily relies on allochtonous organic matter, and its abundance in rivers is generally higher than in lakes, with increasing productivity in higher stream order rivers. The metabolism and trophic level of the water column generally changes along the river as it is affected by illumination, turbulence, turbidity, and the relative abundance of suspended coarse and fine particulate organic matter (CPOM, FPOM, respectively). For example, river metabolism can be heterotrophic both in headwaters (production/respiration, or P/R ratios < 1), which are often characterized by higher turbulence, shaded banks, and higher inputs of CPOM in the riparian zone; and in lower fluvial sections, which are often characterized by higher turbidity levels. Autotrophic metabolism (P/R > 1) can occur in intermediate sections.

In fact, most of the fluvial metabolism generally takes place in the hyporheic zone, which is dominated by microbial benthic activity, mainly through anaerobic processes and heterotrophic consumption of allochthonous dissolved and particulate organic matter. Most of this primary production derives from the terrestrial, emergent, and floating vegetation found in the riparian zone, floodplains, and associated wetlands. Aquatic insects often dominate the invertebrate benthic macrofauna of rivers, and are primary food sources for fish and other vertebrates. In general, production rates of fish in rivers is higher than in lakes, since rivers are characterized by proportionally wider riparian zones and associated transitional systems, where most of the highly diverse and productive benthic habitats are found.

Human Uses

The connection between human civilization and large rivers is witnessed by the numerous human settlements, both extant and archaeological, distributed along their banks or in closely associated systems, such as fertile reclaimed floodplains and wetlands. In general, increasingly larger human settlements developed downstream, as the topography becomes more flattened, and near coastal areas, where estuaries offer preferential access to the sea. For thousands of years, rivers have been used for navigation (transportation, commerce, exploration, and tourism), bathing and other recreational activities, and waste disposal. Rivers have also been extensively utilized as political boundaries and defensive barriers, such as the Mississippi in North America, and the northern border of the Roman Empire along the Danube, respectively, easily controlled by crossing points (bridges). Riverbanks provided caves of gravel and sand for building purposes, and river channels provided sources of food and water, both for drinking (for humans and livestock) and irrigation, from at least 5,000 years ago. Since the 1900s, and most markedly after the 1960s, the green revolution increased human dependency on irrigation for agriculture, which was responsible for 70 percent of the total water used globally by 2020.

Since the ancient Romans, and with a rapid acceleration since the late 1800s to early 1900s, dams and reservoirs have been built as water management tools (hydromodification) for agricultural, industrial, and domestic consumption; for production of hydropower (e.g., watermills, blacksmiths' forges, mining, quarrying, and hydroelectric power facilities); and protection from floods. Channelization is a river engineering procedure that substitutes straight cuts for the meandering river course. Straightening and deepening the river channel increases drainage in lowland agricultural systems or make streams suitable for navigation, especially for larger vessels.

Anthropogenic Impacts

The biodiversity, stability (e.g., flood predictability and sensitivity to headwater disturbing events), and productivity of large river ecosystems largely depend on floodplains and associated habitats, which greatly increase the spatial and temporal heterogeneity of these systems, efficiently store water and nutrients, and provide most of the organic matter that derives from plant and microbial production. Dams and reservoirs drastically disconnect rivers from flood plains and wetlands, thus reducing floodplain habitats and disrupting the natural cycle of inundations. This makes rivers' metabolism entirely dependent on downstream transport from headwaters. Channelization has several negative effects, mainly deriving from the reduction in stream length, increase of flow velocity, and reduction of ecological connectivity with terrestrial ecosystems. Many habitats are lost (riffles, pools, riparian vegetation, and wetlands), and biodiversity is drastically reduced. Other effects include lowering of the water table, increased runoff and erosion, nutrient losses, greater temperature and flow fluctuations, and increased turbidity. The reduction of water retention capacity can also result in increases in frequency and destructivity of downstream flooding events.

Human impacts on transitional flood plain and wetland systems result in overall declines of many fluvial ecosystem services, such as destruction of fisheries, species extinction, reduced nutrient uptake, lower water retention capacity, and higher sensitivity to pollution events. In the attempt of increasing the accessible runoff for human use, by the end of the twentieth century, about two-thirds of fluvial waters were regulated by 800,000 dams. By 2009, there were an estimated 48,000 dams higher than 50 feet (15 meters). Hundreds of thousands of rivers and streams are regulated by levees, embankments, or dikes. Global climate disruption will likely exacerbate these effects, with several models predicting decreased precipitation frequency and increased intensity of precipitation events, which would determine higher fluctuations of downstream flows and major declines of habitat quality of the still functional wetlands and flood plains. Glacial retreat can also reroute meltwater from one river to another, a rare, sudden geological action known as river piracy or stream capture.

Degradation of fluvial waters mainly occurs as eutrophication, siltation, acidification, and introduction of toxic pollutants. Nutrient loadings (e.g., phosphorous and nitrogen) mainly result from organic pollution (e.g., sewage discharges, industrial and agricultural runoff, and nutrient losses in the watershed from harvesting and fire), which cause nutrient enrichment and massive growth of algae and macrophytes. Soil erosion and inputs of suspended sediments result from deforestation and agricultural activities in the watershed, reducing the volume of aquatic habitats and increasing water turbidity. Loadings of hydrogen ions from strong acids result from gases produced by combustion of fossil fuels, which enter the system as fallout or precipitation (rainfall and snowfall), affecting water pH and alkalinity. Direct inputs of toxic substances (e.g., heavy metals, pesticides, chlorinated hydrocarbons, and radioactive materials) from industrial and urban wastes variously affect the biotic components and the metabolism of fluvial ecosystems. All these forms of degradation ultimately affect water quality for human use and consumption.

Management and Restoration

Restoration and/or management of riparian areas and flood plains play a major role for an efficient management of large rivers. In fact, marginal habitats make rivers naturally more resistant and resilient to natural and human-induced disturbances. For this reason, river restoration activities often imply modifications of the channel that encourage development of wetlands and riparian vegetation on buffer strips along the banks; reduce the slope gradient at the edges; and promote channel meandering. In the 2000s and 2010s, dam removals for river restoration have also increased dramatically, with swift ecological changes following: loosened sediments clear in the span of weeks to months and, within a matter of a few years, native migratory fish populations begin to rebound. In 2020, sixty-nine dams were removed, reconnecting more than six hundred rivers in the United States. The effort restored fish populations and stimulated local economies. Even limited restoration or rehabilitation of transitional systems can yield considerable improvements of fluvial ecosystem services. For instance, it has been estimated that watersheds including 5–10 percent of wetlands would experience a 50 percent reduction of peak floods, compared to watersheds without wetlands.

Long-term reduction of the inputs of pollutants implies complex social, political, administrative, and economic problems, especially when reduction of the sources is attempted, and international boundaries are crossed. Treatment of polluted waters is often highly expensive, and most remedial actions aim at diluting and dispersing pollutants.

Bibliography

Allan, J. David, and María M. Castillo. Stream Ecology: Structure and Function of Running Waters. 2nd ed., Springer, 2007.

Biello, David. "The Dam Building Boom: Right Path to Clean Energy?" Yale Environment 360, Yale School of Forestry & Environmental Studies, 23 Feb. 2009, e360.yale.edu/features/the‗dam‗building‗boom‗right‗path‗to‗clean‗energy. Accessed 31 July 2018.

Hassenzahl, David M., et al. Environment. 10th ed., John Wiley & Sons, 2018.

Holsopple, Kara. "River Advocates Call for Removing Old Dams More Quickly." The Alleghany Front, 4 Mar. 2022, www.alleghenyfront.org/american-rivers-dam-removal-infrastructure-law/. Accessed 12 July 2022.

"Irrigation Water Use." The USGS Water Science School, US Geological Survey, US Dept. of the Interior, 26 June 2018, water.usgs.gov/edu/wuir.html. Accessed 31 July 2018.

Lieb, Anna. "The Undamming of America." NOVA Next, WGBH Educational Foundation, 12 Aug. 2015, www.pbs.org/wgbh/nova/next/earth/dam-removals. Accessed 31 July 2018.

Likens, Gene E., editor. River Ecosystem Ecology: A Global Perspective. Elsevier, 2010.

"Sixty-Nine Dams Removed in 2020, Restoring 624 Miles of Rivers Nationwide." American Rivers, 18 Feb. 2021, www.americanrivers.org/conservation-resource/69-dams-removed-in-2020-reconnecting-624-miles-of-rivers-nationwide/. Accessed 12 July 2022.

Thompson, Andrea. "Warming-Driven Glacier Melt Leads to ‘River Piracy.’" Climate Central, 17 Apr. 2017, www.climatecentral.org/news/warming-driven-glacier-melt-river-piracy-21358. Accessed 31 July 2018.