Rivers and Lakes - Their Distribution, Origins, and Forms
Comparative Hydrodynamic Characteristics among River, Reservoir, and Natural Lake Ecosystems
Rivers
Water Level Fluctuations:
Large, rapid, and irregular, primarily driven by precipitation events (e.g., rainfall, snowmelt) and upstream discharges; flooding is common and often unpredictable.
Inflow: the movement of water into a river system, which can vary significantly based on seasonal weather patterns and watershed conditions, contributing to the overall changes in water levels.
Predominantly from overland runoff and groundwater seepage; highly irregular and seasonal, reflecting climatic patterns and land use practices within the watershed. While large groundwater inflows can moderate seasonality, surface runoff remains the primary driver of pulses.
Outflow (Withdrawal): the movement of water out of a river system, which can be influenced by factors such as evaporation, human extraction for agricultural or municipal use, and upstream water management practices. This process is crucial in determining the overall health and sustainability of aquatic ecosystems, as excessive outflow can lead to reduced water levels and affect habitat availability.
Discharge is highly irregular, directly coupled with inflow events and precipitation frequency. This dynamic flow regime influences transport of materials downstream.
Flushing Rates:
Typically rapid, unidirectional, and horizontal. This swift flow minimizes stagnant zones and leads to rapid removal and transport of dissolved and particulate matter through the ecosystem.
Reservoirs
Water Level Fluctuations:
Large and irregular, often controlled by human operations for flood control, hydropower generation, irrigation, and water supply, leading to significant seasonal and even daily fluctuations.
Fresh waters have higher renewal times because
Inflow:
Most runoff enters the reservoir via river tributaries, often high-order streams that carry large volumes of water and sediment. The penetration of these inflows into stratified strata can be complex, often occurring as over-, inter-, or underflows depending on temperature and density differences, and frequently directed along the original riverbed valley.
Outflow (Withdrawal):
Highly irregular due to variable downstream water demands (e.g., hydroelectric power, agriculture, municipal use). Withdrawals can occur from surface layers (epilimnion) or from deeper, often colder and deoxygenated, hypolimnion layers, impacting downstream water quality.
Flushing Rates:
Short and highly variable, ranging from days to several weeks. These rates increase with surface water withdrawals. Hypolimnetic withdrawals can disrupt thermal stratification. Flushing is complex and three-dimensional, influenced by inflow dynamics, basin morphometry, and withdrawal points.
Natural Lakes
Water Level Fluctuations:
Generally small and stable, primarily influenced by long-term climatic patterns, minor precipitation events, and wind-driven seiches. They typically exhibit much less extreme fluctuation compared to rivers and reservoirs.
Inflow:
Runoff enters the lake via tributaries (often lower stream orders) and various diffuse sources such as direct precipitation, shallow groundwater, and overland flow. Penetration into stratified waters is typically small and dispersive due to lower energy inputs and more stable hydrodynamic conditions.
Outflow (Withdrawal):
Relatively stable and constant over longer periods, usually involving surface water discharge via a surface outflow stream or through shallow groundwater pathways.
Flushing Rates:
Long and relatively constant, ranging from one to many years. This slow flushing allows for greater accumulation of dissolved and particulate substances, and the three-dimensional mixing patterns contribute to internal cycling of nutrients.
Freshwaters typically exhibit faster renewal rates because they have a smaller volume, bodies of water like rivers have a higher flow rate. Also becuase water evaporation lifts the water into the atmosphere where it condenses and precipitates back into the water quickly leading to rapid turnover, which significantly enhances the cycling of nutrients and helps maintain the overall health of aquatic ecosystems.
In contrast, larger bodies of water such as lakes may have extended flushing times, which can lead to nutrient stratification and lower oxygen levels in deeper layers.
Where are most lakes located?- Northern North America (most of Canada and Alaska) and northern Eurasia (Scandinavia and much of Siberia) characterized by long, very cold winters, short, cool summers, and significant temperature variations.
Understanding Reservoir Formation and Characteristics
Reservoirs are created predominantly in regions where large natural lakes are sparse or unsuitable for human exploitation (e.g., due to high salinity, extreme depth, or remote location). These regions often lack extensive glacial history or tectonic activity that forms large natural basins.
These regions tend to have:
Warmer average water temperatures, which can influence primary productivity and decomposition rates.
Longer growing seasons, prolonging biological activity within the water column.
Precipitation inputs closely balanced to, or less than, evaporative losses (Wetzel, 1990b), making water storage critical for human use.
The drainage basins of reservoirs are consistently much larger relative to the reservoir surface area than those of most natural lakes. This larger basin-to-lake ared?ea ratio results in greater runoff volumes and higher sediment and nutrient loads.
Reservoirs occur almost always in river valleys and at the base of drainage basins, resulting in a morphometry that is usually dendritic (tree-like), narrow, and elongated. This shape can impede effective mixing and create distinct longitudinal zones within the reservoir.
Biological and Physical Impacts of Reservoir Characteristics
Physical characteristics of reservoirs affect biological processes significantly:
Light and Nutrient Availability:
Reservoirs receive runoff water mainly via high-order streams, which often flow at high energy, leading to significant erosion of soils and streambeds.
This results in large sediment-load carrying capacities, bringing extensive penetration of dissolved and particulate loads into the recipient lake water, increasing turbidity and reducing light penetration, thus limiting primary production in the photic zone.
Runoff Inputs:
Larger and more directly coupled to precipitation events than in most natural lakes. This leads to abrupt changes in inflow, water quality, and sediment transport, creating environmental stress for aquatic organisms.
Nutrient and Sediment Loading:
Characterized by high but irregularly pulsed loading of nutrients (e.g., phosphorus, nitrogen) and sediments. These irregular pulses can disrupt ecosystem stability and lead to episodic eutrophication events.
Fluctuations and Erosion in Reservoirs
Extreme and irregular water level fluctuations in reservoirs occur due to:
Variable flood inflow characteristics from precipitation events impacting large catchment areas.
Land-use practices within the watershed (e.g., deforestation, agriculture) that are not conducive to natural water retention and increase runoff velocity.
Channelization of primary influents, which increases the speed of water delivery to the reservoir and reduces natural flood attenuation.
Flood control efforts that require rapid drawdown and storage of water.
Large irregular water withdrawals for hydropower generation, which can cause diurnal or weekly fluctuations in water levels.
These fluctuations have multiplicative effects on loadings:
Large areas of sediments in the littoral zone can be alternately inundated and exposed. During exposure, organic matter oxidizes, and during inundation, anaerobic conditions can lead to enhanced nutrient release (e.g., iron-bound phosphorus).
Man-made manipulations, such as frequent water level changes and wave action, often prevent productive, stabilizing wetland and littoral flora (e.g., rushes, sedges, submerged macrophytes) from establishing, removing their natural filtering and stabilizing functions.
Erosion and resuspension of floodplain sediments from the barren littoral zone increases nutrient loading from large drainage basin sources, further contributing to internal nutrient cycling.
Surficial sediments shift between aerobic and anaerobic conditions due to changing water levels, enhancing the release of redox-sensitive nutrients like phosphate and ammonium, which become soluble under anoxic conditions.
The reduction or elimination of wetland and littoral communities significantly decreases effective nutrient and physical sieving capacities (the ability to filter and retain sediments and nutrients), a function typically found in natural lakes.
Comparison between Reservoirs and Natural Lakes
Natural Lakes
Shoreline erosion is typically small and localized to wind-generated currents, with wave action largely determined by fetch and wind speed. The presence of established littoral vegetation also minimizes erosion.
Basin morphology is relatively stable compared to reservoirs, with less drastic changes in depth, shoreline configuration, and sediment deposition over time. This stability supports more mature and complex ecological communities.
Summary of Water Bodies Findings
Approximately 40% of the total volume of fresh water is contained in great lakes basins; however, most lakes and reservoirs are much smaller (less than in depth) and concentrated in subarctic and temperate regions of the Northern Hemisphere, largely due to extensive glaciation during the last ice age. These countless smaller bodies, despite their individual sizes, collectively play a crucial role in regional hydrology and biodiversity.
Rivers account for only 0.0001% of the Earth's water yet are significant transporters of dissolved and particulate matter from land to sea, playing a critical role in global biogeochemical cycles:
Precipitation enters drainage basins and is either moved through surface runoff, subsurface flow, or temporarily stored in soils, vegetation, or shallow groundwater, before contributing to streamflow.
Stream flow and discharge are directly related to the drainage area, rainfall intensity, and variations in elevational gradients and soil characteristics. Higher gradients and impermeable soils lead to faster and greater discharge. The velocity of a river can be estimated using empirical formulas like the Manning equation, where , with as velocity, as Manning's roughness coefficient, as hydraulic radius, and as channel slope.
River channels can move laterally within the floodplain due to erosion on the outer banks of meanders and redeposition of sediment on the inner banks, leading to the formation of oxbow lakes over time.
Forms of river channels include straight, meandering, braided, and anastomosing, varying widely with geology, sediment load, discharge rate, width-depth ratios, and channel slope. Meandering rivers are common in low-gradient areas, while braided rivers are typical in areas with high sediment loads and fluctuating discharge.
Most natural lakes formed as a result of catastrophic events, including:
Tectonic Activity:
Tectonic lake basins are depressions formed by displacements of the Earth's crust, often resulting from faulting movements (e.g., rift valleys) that create very deep lakes (e.g., Lake Baikal, the African Great Lakes). Uplifting of the Earth's crust can also create various lake basins by blocking drainage.
Volcanic Activity:
Small, deep maar lakes can form in volcanic cones from explosive eruptions where magma interacts with groundwater. Caldera lakes can result from the collapse of partially emptied magmatic chambers after a major eruption, forming large, circular depressions (e.g., Crater Lake).
Landslides:
Temporary or permanent lakes may form when large masses of landfall material block stream valleys, impounding water upstream. These lakes can pose flood hazards if the natural dam fails.
Glacial Activity:
A significant agent of lake formation, with many lakes resulting from glacial outwash morainal deposits (e.g., damming valleys), melted ice blocks creating kettle lakes (e.g., in pitted outwash plains), and scoured lake basins leading to cirque lakes (bowl-shaped depressions in mountains) or large finger lakes (e.g., in New York).
Natural lakes can also form through gradual processes:
Solution Lakes:
Result from the gradual dissolution of soluble rock, such as limestone (karst topography), gypsum, or salt beds, forming depressions that fill with water. These are common in many humid subtropical regions.
Erosional/Depositional Activity:
Can isolate depressions to form lakes. This includes fluviatile (riverine) processes carving out or depositing sediments, aeolian (wind) erosion creating deflation basins, or coastal processes forming lagoons behind barrier islands.
Wind Erosion:
Can create shallow depressions (playas or pans) that temporarily contain water, especially in arid and semi-arid regions where fine sediments are easily mobilized.
Coastal Formation:
Lakes can form due to longshore currents depositing sediments (e.g., spits, barrier bars) that enclose parts of the sea or estuaries, creating lagoons or coastal lakes. These are often brackish or saline.
Reservoir Creation:
Human-made lakes formed by damming river valleys, often designed for specific purposes. They are generally short-lived in geological terms due to high sedimentation rates from their large, erosive drainage basins.
Lake basin morphology varies greatly; most approximating an elliptic sinusoid shape, with the mean depth about half of the maximum depth (). This relationship is important for understanding how light penetrates and mixes within the water column.
The morphology of a lake basin significantly impacts its physical, chemical, and biological properties:
Morphometry influences sediment-water interactions (e.g., nutrient exchange rates from the benthos to the water column) and lake productivity, especially in productive littoral communities where light can reach the bottom and support rooted vegetation.
Shallow lakes have more sediment area per unit volume of water and are generally more productive than deep lakes, largely because they can support a greater abundance of aquatic plants and algae due to increased light availability (littoral zone- near shore area), which enhances primary production. Conversely, deep lakes are often stratified and have lower productivity due to limited light penetration, which restricts the growth of photosynthetic organisms at greater depths.
Reservoirs structured by damming exhibit three distinct longitudinal zones due to varying flow characteristics and sediment deposition:
Riverine Zone:
Near the inflow, characterized by high advective water transport, lotic (flowing) conditions, high particulate turbidity, and generally high oxygen levels. Biota are adapted to flowing water.
Transition Zone:
As water velocities decrease, sedimentation increases, and thermal stratification may begin to develop. This zone marks a shift from riverine to more lacustrine conditions, with a mix of lotic and lentic species.
Lacustrine Zone:
Closest to the dam, exhibiting characteristics similar to natural lakes, including slower water movement, often deeper and more pronounced thermal stratification, and a well-developed pelagic (open-water) community. Sedimentation rates are generally lower here compared to the transition zone.
Examining geomorphological characteristics among river, reservoir, and natural lake ecosystems clarifies differences:
Rivers are typically shallow, dynamic systems constantly shifting between erosional and depositional states, thus having unstable boundaries.
Natural lakes are generally much more morphologically stable compared to both reservoirs and rivers concerning depth, shoreline development, sediment loading, turbidity, and sediment deposition patterns, leading to more stable ecological conditions.