The Physical and Chemical Setting

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Stream and river biota evolved in response to, and in concert with, the physical and chemical setting. Although traditionally the domain of hydrologists, geo-morphologists, and chemists, study of processes driving the physical and chemical templates have been embraced by stream ecologists for interpreting patterns in organis-mic distributions and lotic ecosystem structure and function. From a purely physical perspective, the primary function of rivers is to transfer runoff and move weathering products away from terrestrial portions of the Earth for delivery to the oceans. Despite tremendous variability in the morphology and behavior of rivers, each results from the interaction between geomorphic and hydrologic processes. These processes and their effect on river morphology are summarized, followed by a discussion of major physical (current, substrate, and temperature) and chemical factors that affect the functioning of river ecosystems and the adaptations of stream organisms.

Hydrologic Processes

The total amount of the Earth's water does not change, and is continuously recycled among various storage compartments within the biosphere in a process referred to as the hydrologic cycle (Figure 1). The cycle involves evaporation from land and evapotranspiration from terrestrial vegetation driven by solar energy, cloud formation, and precipitation.

Annual global precipitation averages about 100 cm, but the majority evaporates and little falls directly into streams. The remainder either infiltrates into the soil or becomes surface runoff. The relative contributions of different pathways by which water enters streams and rivers varies with climate, geology, watershed physiography, soils, vegetation, and land use.

Water that infiltrates becomes groundwater, which makes up the largest supply of unfrozen freshwater. Groundwater discharges gradually to stream channels through springs or direct seepage when a channel intersects the groundwater table. Baseflow describes the proportion of total stream flow contributed from ground-water, and sustains streams during periods of little or no precipitation. Running waters may be categorized by the balance and timing of stormflow versus baseflow. Ephemeral streams carry water only in the wettest years and never intersect the water table. Intermittent streams flow predictably every year only when they

Cloud formation

Rain clouds



Figure 1 The hydrologic cycle. From Stream Corridor Restoration: Principles, Processes, and Practices, 10/98, by the Federal Interagency Stream Restoration Working Group (FISRWG).

receive surface runoff (Figure 2). Perennial streams flow continuously during wet and dry periods, receiving both stormflow and baseflow. The duration, timing, and predictability of flow greatly affect the composition and life-history attributes of stream communities.

Stream and river discharge, the most fundamental of hydrological measurements, describes the volume of water passing a channel cross-section per unit time.

Any increase in discharge must result in an increase in channel width, depth, velocity, or some combination of these. Discharge increases in a downstream direction through tributary inputs and groundwater addition and is accompanied by increases in channel width, depth, and velocity. An estimated 35 000 km3 of water is discharged annually by rivers to the world's oceans, with the Amazon River alone accounting for nearly 15% of the total.

Hydrographs depict changes in discharge over time. Individual storm events display a steep rising limb from direct runoff, a peak, and a gradually falling recession limb as the stream returns to baseflow conditions (Figure 3). Variability in the shapes of hydrographs among streams reflects differences in the climatic, geo-morphic, and geologic attributes of their watersheds and differences in the distribution of runoff sources.

Discharge records of sufficient duration allow prediction of the magnitude and frequency of flood events for a given river and year. Recurrence interval (T, in years) for an individual flood may be estimated as

T — (n + 1)/m where n is the number of years of record, and m is ranked magnitude of the flood over the period of record, with the largest event scored as m — 1.

The reciprocal of T is the exceedance probability, which describes that statistical likelihood that a certain

Figure 2 An intermittent stream at 3.4 km elevation in the Andes Mountains in Chile, bordered by riparian vegetation of herbs and grasses. Intermittent streams are often important in exporting invertebrates and organic detritus to downstream fish-bearing reaches.

Lag time

Lag time

Recession y limb

Time (days)



Recession y limb

Time of rise

Time (days)

Figure 3 Stream hydrograph from a rainstorm event. From Stream Corridor Restoration: Principles, Processes, and Practices, 10/98, by the Federal Interagency Stream Restoration Working Group (FISRWG).

discharge will be equaled or exceeded in any given year. Thus a 1-in-100-year flood has a probability of 1% of occurring in any given year. The probability that a 100-year flood will occur in a river is the same every year, regardless of how long it has been since the last 100-year flood. Recurrence interval information provides an extremely important context for studies of lotic organisms.

Geomorphic Processes

Discharge and sediment supply represent the physical energy and matter that move through river systems, and channel form and profile change over time to accommodate the energy and matter delivered to it. Three primary geomorphic processes, including erosion, transport, and deposition, supply sediment to streams and rivers. Physical/chemical weathering of bedrock and soils, together with channel, bank, and floodplain erosion account for short-and long-term lotic sediment supply. Initiation of sediment movement in the channel is a function of drag and lift forces exerted on sedimentary particles. The greater the velocity and shear stress exerted on the streambed, the greater the grain size that can be entrained. Stream competence and stream capacity refer to the largest grain size moved by a given set of flow and the total amount of sediment that can be transported, respectively.

Coarse sediment moves along the stream/river bottom as bedload, and fine sediment moves downstream in the water column as suspended load. The suspended load, or turbidity, screens out light and scours off organisms attached to the bottom while the organic fraction serves as the food resource for invertebrate filtering collectors. Whereas sediments may be temporarily deposited within mid-channel or point bars, longer-term storage occurs on floodplains and elevated alluvial terraces.

Channel Morphology

Within a reach, channel cross sections reflect the interaction between bank materials and flow and vary from symmetrical in riffles to asymmetrical in pools as flow meanders. Bankfull discharge, when discharge just fills the entire channel cross-section, occurs every 1.5-2 years on average in unregulated systems. Erodible banks lead to wide shallow rivers dominated by bedload, while resistant banks produce narrow, deep channels transporting high suspended loads.

Channel pattern is described by its sinuosity (amount of curvature) and thread (multiple channel braiding). Sinuosity index is measured as channel length along the thalweg (deepest portion of the channel), divided by valley length. If the index exceeds 1.5, the stream/river is classified as meandering. Erosion of the channel bank carves the river bends, with the fastest current at the outside of the bend where the bank erodes. The greater the curve, the faster the water flows around the bend, deflecting to the other bank and forming the next curve. This pattern repeats downstream, creating regular swings in the river with a meander wavelength approximately 11 times the channel width.

Riffles are topographic high spots along the channel composed of the coarsest bedload sediments transported by the river, and with a water surface slope that is steeper than the mean stream gradient at low flow (Figure 4). They are typically spaced every five to seven channel widths. Pools are topographic depressions with fine sediments and reduced velocity.

The longitudinal profile of a river is relatively stable over time, adjusting slowly to discharge and sediment supply. The profile is generally concave, with a steep gradient in its headwaters, and a gentle gradient at its






Thalweg line


Thalweg line

Riffle or cross over

Figure 4 Riffle and pool sequences in straight and sinuous streams. From Stream Corridor Restoration: Principles, Processes, and Practices, 10/98, by the Federal Interagency Stream Restoration Working Group (FISRWG).

Riffle or cross over

Figure 4 Riffle and pool sequences in straight and sinuous streams. From Stream Corridor Restoration: Principles, Processes, and Practices, 10/98, by the Federal Interagency Stream Restoration Working Group (FISRWG).

mouth. The concavity reflects the adjustment between climate and tectonic setting (land relief and base level) and geology, which controls sediment supply and resistance to erosion. Base level describes the limit to which a river cannot erode its channel. For streams emptying into the ocean, this is sea level.

Within a drainage basin, stream channels and their networks grow in size and complexity in a downstream direction as described by stream order (Figure 5). A first-order stream lacks permanently flowing upstream tributaries and order number increases only where two stream of equal order join together. Employing this system, the Mississippi and the Nile Rivers at their mouths are order 10. There are usually

Figure 5 Ordering of stream segments within a drainage network. From Stream Corridor Restoration: Principles, Processes, and Practices, 10/98, by the Federal Interagency Stream Restoration Working Group (FISRWG).

3-4 times as many streams of order n - 1 as of order n, each of which is roughly half as long, and drains a little more than one-fifth of the land area. In the United States, nearly half of the approximately 5 200 000 km total river length are first order. As discussed later, many features of stream ecosystem structure and function are correlated with stream order.

Drainage basins, or watersheds, are the total area of land draining water, sediment, and dissolved materials to a common outlet. Watersheds occur at multiple scales, ranging from the largest river basins to first-order watersheds measuring only a few hectares in size. Larger watersheds are comprised of smaller watersheds and stream segments in a nested hierarchy of ecosystem units. The size and shape of the watershed, and the pattern of the drainage network within the watershed, exerts a strong influence on the flux of energy, matter, and organisms in river systems. Because some movement of energy, matter, and organisms move across and through landscapes independently of drainage basins, a more complete perspective of stream ecology requires consideration of landscape ecology.

Physical Factors Current

Current (ms_1 of flow) is the central defining physical variable in running-water systems. Velocity and associated flow forces exert major effects on stream organisms. Current shapes the nature of the substrate, delivers dissolved oxygen, nutrients, and food, removes waste materials, and exerts direct physical forces on organisms on streambed and in the water column, resulting, for example, in the dislodgement and displacement of organisms downstream. Current velocity, which rarely exceeds 3ms~ in running waters, is influenced by the river slope, average flow depth, and resistance of bed and bank materials.

Flow in running waters is complex and highly variable in space and time. At a given velocity, flow may be laminar, moving in parallel layers which slide past each other at differing speeds with little mixing, or turbulent, where flow is chaotic and vertically mixed. The dimen-sionless Reynolds number, the ratio of inertial to viscous forces, predicts the occurrence of laminar versus turbulent flow. High inertia promotes turbulence. Viscosity is the resistance of water to deformation, due to coherence of molecules. At Reynolds numbers <500, flow is laminar; at >2000 flow is turbulent with intermediate values transitional. Although laminar flow is rare in running waters, microenvironments may contain laminar flow environments, even within turbulent, high-flow settings.

In cross section, a vertical velocity gradient decreases exponentially with depth. Highest velocities are at the surface where friction is least, and zero at the deepest point of the bottom where friction is the greatest. Mean current velocity is at about 60% of the depth from the surface to bottom. A boundary layer extends from the streambed to a depth where velocity is no longer reduced by friction and a thin viscous sublayer of laminar flow exists at its base.

Microorganisms and small benthic macroinvertebrates experience shelter from fluid forces within the sublayer. However, most stream organisms must contend with complex, turbulent flow where they exhibit a variety of morphological and behavioral adaptations for reducing drag and lift. Adaptations of macroinvertebrates and fishes may include small size, dorsoventral flattening to reduce exposure to current, streamlining to reduce current drag, the development of silk, claws, hooks, suckers, and friction pads as holdfasts, and behavioral movement away from high-velocity areas.


In running waters, substrate provides food or a surface where food accumulates, a refuge from flow and predators, a location for carrying out activities such as resting, reproduction, and movement, and material for construction of cases and tubes. Algal growth, invertebrate growth and development, and fish egg incubation largely occur on or within the substrate. Substrate includes both inorganic and organic materials, often in a heterogeneous mixture. Mineral composition of the substrate is determined by parent geology, modified by the current. Organic materials include aquatic plants and terrestrial inputs from the surrounding catchment ranging from minute fragments and leaves to fallen trees (Figure 6).

Inorganic and organic materials are often classified by size according the Wenthworth scale (Table 1). A broad classification of organic materials is discussed in Rivers and Streams: Ecosystem Dynamics and Integrating Paradigms. Organic particles <1mm in diameter and >0.45 mm (fine particulate organic matter or FPOM) often function as

Woody Detritus Streams
Figure 6 Small headwater stream in old-growth Douglas-fir forest in Oregon, showing large woody debris spanning the channel. This spanner log forms a retention structure for organic detritus and sediment as well as refugia and habitat when the channel is inundated by high flows.
Table 1 Size categories of inorganic substrates in streams and rivers

Size category

Particle diameter (range in mm)





















Very coarse








Very Fine




Modified from Cummins KW (1962) An evaluation of some techniques for the collection and analysis of benthic samples with special emphasis on lotic waters. American Midland Naturalist 67: 477-504.

Modified from Cummins KW (1962) An evaluation of some techniques for the collection and analysis of benthic samples with special emphasis on lotic waters. American Midland Naturalist 67: 477-504.

food rather than substrate, and larger organic materials (CPOM) serve as substrate or food, for example, for litter-feeding invertebrates (Figure 7). Other substrate attributes, including shape, surface texture, sorting, and stability, are also determinants of benthic community structure, but these are less easily quantified. In general, larger, more stable rocks support greater diversity and numbers of individual organisms than smaller rocks, but smaller rocks with a higher ratio of surface area to volume support higher densities.

Riparian Zones And Litter

Figure 7 Accumulation of leaf litter in a second-order stream in Oregon (USA) flowing through a second-growth forest with a red alder riparian zone. The litter that is retained at the leading edge of the cobbles provides the major food resource for stream invertebrate shredders and habitat for other invertebrates.

Figure 7 Accumulation of leaf litter in a second-order stream in Oregon (USA) flowing through a second-growth forest with a red alder riparian zone. The litter that is retained at the leading edge of the cobbles provides the major food resource for stream invertebrate shredders and habitat for other invertebrates.

Evaluation of the ecological role of substrate is difficult because of its heterogeneity and covariance with velocity and oxygen supply. Heterogeneity is expressed along the length of a river as decreasing particle size in a downstream direction and at a reach scale as pool and riffle sequences, meandering, and point bar development. Substrate embeddedness describes the degree to which larger sediments, such as cobbles, are surrounded or covered by fine sand and silt. Significant embeddedness reduces streambed surface area and organic matter storage, the flow of oxygen and nutrients to incubating fish eggs and aquatic invertebrates, and entrance to and movement within the streambed by invertebrates.


Temperature affects all life processes, including those in running waters. For example, decomposition, primary production and community respiration, and nutrient cycling are all temperature dependent. Most stream organisms are ectothermic, and their metabolism, growth rates, life cycles, and overall productivities are all temperature dependent. Annual temperature changes often serve as environmental cues for and/or regulate life-history events of invertebrates and fishes, especially emergence or spawning. The temperature regime sets limits to where species can live, and many species are adapted to certain thermal regimes. Increasing water temperature decreases dissolved oxygen solubility at the same time that it increases metabolic demand. Thus preferences of such organisms as salmonid fishes for cold water may have as much to do with temperature effects on oxygen availability as with effects of temperature per se.

Stream temperature is the net result of heat exchange via (1) net solar radiation, which reflects direct beam solar radiation, modified by cloud cover, day length, sun angle, vegetation, and topographic shading; (2) evaporation and convection, which are affected by vapor pressure and air temperature differentials as well as wind speed;

(3) conduction, or heat exchange with streambed; and

(4) advection from upstreamwater inputs, including groundwater and tributaries. On a diel basis, stream temperature varies less than air temperature because of the high specific heat of water. The greatest daily fluxes occur in summer in temperate regions, with a minimum before dawn. These fluxes are greatly affected by canopy cover and contributions of groundwater, which usually enters the channel at a temperature within 1 °C of mean annual air temperature. At the catchment level, daily temperature flux increases with distance from the headwaters, with a maximum in mid-order segments. Thermal stratification is rare except in large rivers and at tributary junctions. Seasonal variations in temperature mirror trends in mean monthly air temperature. The timing of the summer maximum often lags the timing of maximum solar radiation. Year-to-year variation in monthly temperatures is low, typically less than

2 °C. The annual temperature range of temperate streams is generally 0-25 °C, and 0-40 °C in intermittent desert streams. The lower Amazon River is always within one or two degrees of 29 °C. Extremes in temperature occur in hot water springs, which can exceed 80 °C, and in subarctic and arctic streams that may completely freeze in winter. Surface freezing is usually prevented by snow and ice bridging, but underwater ice may form on streambeds as anchor ice or in the water column as slush or frazil.

Stream ecologists often evaluate temperature effects on stream organisms and ecosystem processes on the basis of degree-day accumulation rather than temperature maxima or minima. Degree days, which are calculated by summing daily mean temperatures above 0 °C, can differ among streams with similar maximum or minimum average daily temperatures. Such differences can affect voltinism (number of annual generations) of some species of aquatic insects.

Water Chemistry

Constituents of river water can be divided into five categories, which include dissolved gases, dissolved inorganic ions and compounds, particulate inorganic material, parti-culate organic material, and dissolved organic ions and compounds. Dissolved gases include oxygen, carbon dioxide, and nitrogen. Dissolved inorganic ions and compounds include major and minor ion groups and trace elements, such as copper, zinc, iron, and aluminum which occur in minute quantities. Nitrogen and phosphorus are minor ions, which function as nutrients essential to plant and animal growth. Major ion groups include cations of calcium, magnesium, sodium, and potassium and the anions bicarbonate, sulfate, and chloride.

The pH, which measures hydrogen ion activity, is affected by concentrations of dissolved gases and major ions and determines the solubility and biological availability of nutrients and heavy metals. Hardness is a measure of calcium and magnesium concentrations normally used to assess the quality of water supplies. Hardness is associated with, but not identical to alkalinity, which measures the ability of streamwater to absorb hydrogen ions, thus buffering changes in pH. Alkalinity is primarily due to bicarbonate and carbonate ions. Total dissolved solids, the sum of the concentrations of major cations and anions, are often estimated as specific conductance. Hardness, alkalinity, and ionic concentrations are frequently positively correlated with stream productivity and taxonomic richness. Particulate inorganic and organic materials together make up the suspended load in lotic systems, and contribute to turbidity.

Carbon dioxide and oxygen are the most biologically important dissolved gases. Diffusion from the atmosphere maintains concentrations of both oxygen (O2) and carbon dioxide (CO2) in streams at close to equilibrium.

However, CO2 is more soluble in water than is O2, which is 30 times less available in water than air. Groundwater and sites of organic matter decomposition are low in O2 and enriched in CO2. Photosynthesis and respiration can alter diel concentrations of oxygen and carbon dioxide in productive systems, with O2 elevated and CO2 reduced during day, and the reverse occurring at night. If production is high relative to diffusion, diel changes in O2 are used to estimate photosynthesis and respiration. Because current and turbulence continually renew O2 supply, its concentrations are problematic for stream organisms only in sites severely contaminated with organic pollutants or through a combination of high temperatures, drought, and dense populations of aquatic plants. Low O2 concentrations are better tolerated by stream animals at faster current speeds.

Typical rivers have been described as essentially dilute calcium bicarbonate solutions dominated by a few cations and anions. The considerable natural spatial variability in lotic chemistry largely reflects the type ofrocks available for weathering and the amount, chemical composition, and distribution of precipitation. For example, total dissolved solids are approximately twice as great in rivers draining sedimentary terrain compared with igneous and meta-morphic rock. Most rivers contain 0.01-0.02% dissolved minerals, about 1/20-1/40th the salt concentration of the oceans, with an average concentration of 100 mg l_ . Generally >50% of this is bicarbonate and 10-30% is chloride and sulfate. River water contains more dissolved solids than does rainwater, because of evaporation, weathering, and anthropogenic inputs. Rainwater, although nearly pure, contains dissolved minerals from dust particles and droplets of ocean spray.

Rainwater is also naturally acidic due to atmospheric carbon dioxide dissolving in the water droplets, forming a weak carbonic acid (H2CO3). In catchments with hard rocks resistant to weathering, little buffering capacity, or where decaying plant matter is abundant, streamwater can be acidic even in absence of pollution. Water percolating through the soil enters the stream and is enriched with CO2 from plant and microbial respiration and forms carbonic acid. The carbonic acid dissolves the calcium carbonate in rocks, producing calcium bicarbonate, which is soluble in water and the source of carbon atoms for aquatic photosynthesis. The dissolution of calcium carbonate increases the amount of stream calcium and bicarbonate ions and the latter dissociates to carbonate ions. At equilibrium, bicarbonate and carbonate ions dissociate, forming hydroxyl ions and resulting in weak alkaline waters, with a pH > 7. At equilibrium, water resists changes in pH because the addition of hydrogen ions is neutralized by the hydroxyl ions formed by dissociation of bicarbonate and carbonate, and added hydroxyl ions react with bicarbonate to form carbonate and water. Thus the buffering capacity of a stream is largely determined by its calcium bicarbonate content. The pH of most natural running waters ranges between 6.5 and 8.5, with values below 5 or above 9 being harmful to most stream organisms. Industrially derived sulfuric and nitric acids have seriously lowered pH in surface waters of large areas of Europe and North America, resulting in reduced species diversity and density.

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