Delta plain (Fig. 1) is a vast mosaic of interdistributary
swamps and marshes encased by distributary and tidal channels (caños). These
interdistributary basins can be thought of as islands in the form of plates that are
slightly elevated along the borders and are flat to slightly depressed to slightly mounded
in the center. These vast interdistributary basins (Photos 1,
2, 3, 4, 5) are seasonally to
perennially flooded, and are typically underlain by mud in the upper and southeastern
delta and peat in the central and northwestern delta (Fig. 2).
Interdistributary basins vary from densely forested to herbaceous with all manner of
gradation between, depending on proximity to major caños, duration of flooding, and
degree of salinity (Photos 6, 7,
8, 9, 1, 2, 3, 4, 5, 10, 11).
Although it is possible to distinguish fluvial and tidal channels within the delta, essentially all channels are influenced by river discharge during the wet season (May through September)and by tidal oscillations during the dry season. The caños vary from meandering, relatively narrow fluvial channels in the upper delta to straight, broad tidal channels in the lower delta. Several major caños are deflected to the northwest in the coastal region under the influence of the suspended-sediment-rich, northwest flowing Guayana Current and associated mudcape development (Fig. 1). The two major distributaries (Grande and Manamo) broaden near the coast, transforming to estuaries. Rio Grande is by far the primary distributary channel transporting nearly 90 percent of the Orinoco River water and sediment discharge. Caños Araguao, Mariusa, Macareo and Manamo are secondary distributary channels, whereas Caños Pedernales, Capure are examples of third-order distributary channels (Fig. 3). Largely as the result of tides, there is an extensive network of channels that generally oriented parallel to the coast that hydraulically link the principal distributary channels (Fig. 3).
The major caños are bordered by levees that form distinct topographic features in the upper delta plain (Photos 12, 13, 14) but gradually diminish and finally disappear towards the coast. The levees are typically the highest features in the delta plain, but are regularly overtopped during flood stage, and serve to retain water in the interdistributary basins as flood stage recedes. Human activity is typically concentrated on these elevated, relatively well-drained landforms (Photos 13, 14).
Cursory inspection of a map or satellite mosaic of the delta reveals two distinct, roughly fan-shaped delta sectors: the southern and southeastern sector, which comprises a complex of anastomosing channels and islands, and a central and northwestern sector which contains relatively straight, parallel channels that subdivide broad wetlands (Fig. 4). The geomorphology, hydrology, and ecology of the southeastern (Rio Grande) sector is fundamentally controlled by riverine and tidal process, and is an estuary. The central and northeastern sector is fundamentally controlled by tides and rainfall, and to a lesser extent by riverine processes, and can be characterized as coastal plain tidal swamp and marsh systems (Photo 9). The differences in geomorphology between the southeastern and northwestern delta sectors reflect the uneven discharge of Orinoco River water and sediment across the delta (Fig. 5), which in turn is largely controlled by the tectonic setting (Fig. 6).
The delta plain can also be subdivided into coast-parallel riverine (upper), river and tide (middle), and tidal (lower) sectors (Fig. 7). The upper delta geomorphology, hydrology, and ecology are fundamentally controlled by river discharge and, to a lesser degree, local rainfall. It is generally 7 to 2.5 m above mean sea level (amsl), seasonally inundated, and naturally covered with deciduous forests (although much has been cleared by humans)(Fig. 16). The substrate is a complex mixture of clay, silt, and sand with only minor peat. The geomorphology, hydrology, and ecology of the middle (fluvial-marine) delta are the result of complex interaction among river discharge, tides, and local rainfall.. The middle delta plain is 2.5 to <1 m amsl, it is flooded for the majority of the year, and is covered by evergreen and palm forests (Photos 9, 2, 4, 5). The middle delta-plain substrate is a mixture of mud and peat. The geomorphology, hydrology, and ecology of the lower delta are primarily controlled by tides but also significantly influenced by the river and local rainfall. The lower delta plain is generally 1 to -1 m above and below sea level, perennially flooded, and is covered by evergreen shade forests, grasses and sedges with mangrove along the coast and caños (Photos 15, 16, 17) . The lower delta-plain substrate is mostly peat but mud, silt and sand (cheniers) are significant in some areas.
The Orinoco Coast, is much like the Guiana coast to the south, with alternating estuaries and rounded promontories (Fig. 1). The estuaries reflect the influence of riverine and tidal processes, whereas the mudcapes reflect the influence of the strong, northwest-directed littoral currents (Fig. 8, Photo 18). Although most of the coast is prograding seaward, some portions are eroding landward (Photo 15), which is natural in major delta systems. Features such as relict beach ridges and mudcapes located inland (Figs. 1, 2) provide a clear indication that the Orinoco Delta has prograded seaward during the late Holocene. Delta progradation is primarily by mud flat accretion and eventual stabilization by mangrove (Photos 17, 19, 20) rather than advancement of promontories at mouths of major distributaries, such as in the Mississippi, Nile, and Danube Deltas.
The climate of the Orinoco River basin and delta is tropical with a pronounced wet and dry season. Rainfall depths typically range from 1200 to 3600 mm per year and are greater to the south, in the Guyana region (Fig. 9). Orinoco basin climate is controlled by the Intertropical Convergence Zone (ITCZ), which is the latitudinal belt along the equator where the easterly Trade Winds of both hemispheres converge, producing warm, humid, unstable air masses that generate yield large volumes of rainfall. The ITCZ seasonally migrates across ~15 degrees latitude, and because the Orinoco basin lies along the northern boundary of this migration belt, a marked wet (June through and November) and dry season (December through April) characterize the Orinoco drainage basin. During the dry season the basin receives only 10 to 15 percent of the annual precipitation. This pattern of marked wet and dry seasons induces a pronounced hydrologic cycle in the river and delta (Fig. 10), which, in turn produces distinct sedimentologic geomorphic, and ecologic cycles. In the delta, however, especially near the coast, light to medium intensity rainfall commonly occurs during the dry season (Fig. 11).
Rainfall volumes are a major environmental factor in the Orinoco Delta, and are a major contributor to surface water inflow to the vast interdistributary basins. Rainfall varies significantly across the delta, ranging from generally ~1500 mm near the delta apex to ~2600 mm per year near the coast (Figs. 9, 12). Although there is a pronounced dry season, rainfall typically exceed 100 mm per month throughout the year in the lower delta, and therefore can be critical to the hydrology during low river stage.
Rainfall data show that sustained intense rainfall events, such as those induced by hurricanes, are not characteristic of the delta. As expected in a tropical environment removed from the track of major tropical disturbances, rainfall is common, but generally not intense; when it is intense, it is of short duration.
Temperatures in the delta region are remarkably homogeneous throughout the year (Fig. 13), in which monthly averages at the Tucupita weather station range from 24.8 to 26.7 degrees Celsius. Winds are generally stronger in the winter, but may be strong any time of year (Fig. 14). Maximum northeast trade-wind energies occur in February to April and this sustained wind activity induces a peak in nearshore wave energy. Short-term, high-velocity winds are associated with hurricanes and tropical storms.
The delta is situated near the southern limit of hurricanes that track from the central Atlantic into the Caribbean region. Therefore hurricanes are not a major climatic influence on delta processes, but occasionally induce high winds, heavy rains, and high-energy waves in the region.
Impact of Volcán Dam
Deltas are transitional terrestrial/marine ecosystems that are the product of interaction and balance of numerous atmospheric, geologic, hydrologic, and biologic processes. Therefore human activity has profound impacts on the ecosystem integrity of these environmental systems. As a consequence, human activity has become a major geomorphic process in many deltas and in some cases, the principle factor. Construction of Volcán Dam along Caño Manamo has significantly influenced the hydrology and ecology of the northwestern Orinoco Delta. In addition, clearing of forests in the upper delta for agriculture, grazing, and human habitation has also significantly impacted delta ecosystems.
Volcán Dam was constructed across Caño Manamo (Fig. 3) in 1966 and 1967 to (1) protect Tucupita from flooding, (2) to expand agriculture in the delta by controlling flood regimes and provide the opportunity for drainage of soils, and (3) raise water levels in Rio Grande to enhance commercial navigation . Discharge through Caño Manamo prior to dam construction was generally between 3500 and 8000 m3/sec, with a minimum discharge of 800 m3/sec. Sediment discharge was estimated to be 25 x 106 tons/yr. Since dam construction, Caño Manamo water discharge has been regulated at 150 to 250 m3/sec, representing a reduction from 10 to 0.5 percent of the total Orinoco discharge. Sediment discharge through upper Caño Manamo has been essentially reduced to zero so that only upland streams entering from the west, e.g. Río Morichal Largo and Río Tigre, discharge sediment into lower Caño Manamo.
Volcán Dam has been effective in controlling flooding in Tucupita, the major city of the region. Expansion of agriculture in the delta plain, however, was unsuccessful because of high pyrite content in the wetland soils, which, when drained converts to sulfuric acid, making the soils inhospitable for agriculture, and even native plants. There are many impacts associated with modification of the natural water and sediment discharge regime by Volcán Dam. The delta is a transitional aquatic/terrestrial ecosystem complex, which means that ecosystems and processes are strongly interrelated, and therefore the impacts of Volcán dam tend to cascade from one set of processes and ecosystem to others. The impacts of Volcán Dam water regulation include:
* Upstream incursion of estuarine (brackish water) has induced a marked increase in upstream tidal flow of marine waters. Cascading effects of the landward salt-water incursion include: expansion of mangrove forests upstream, and upstream replacement by brackish and marine fish populations. Moreover, expansion of mangroves has significantly increased rates of sediment entrapment which in turn has accelerated expansion of channel islands in the lower Caño Manamo and infilling of Boca de Guanipa.
* Increase in water temperatures and decrease in dissolved oxygen content in the caños.
* Reduction of sediment discharge through Caño Manamo and onto the surrounding delta plain and coast.
* Clogging of caños by floating vegetation (especially water hyacinth) (Photo 13). Prior to dam construction, these caños were flushed by annual floods, which maintained open channels (such as occur in Caños Macareo, Araguao, Mariusa, etc.). Now, water hyacinth in Caños Pedernales and Capure can be so dense that navigation is impeded. Caño Tucupita, is almost completely infilled (fig. 27o). Moreover, upon dying, the floating vegetation settles to the bottom of caños, greatly accelerating infilling and loss of sandy channel-bottom habitat for fish and invertebrates.
* Enhanced development of bars at the seaward edge of Boca de Guanipa and filling of broad zones in main channels.
The marked reduction in water and sediment discharge to the northwestern delta (Photo 21) may induce wetland loss as the delta plain subsides and open water conditions prevail, as in the Mississippi Delta. However, bioaccumulation associated with peat development in the Orinoco Delta may be offset the reduction in mineral sediment influx. Peat marshes, however, support different ecosystems than terrigenous wetlands.
Large portions of the upper delta have been cleared for grazing, agriculture, and habitation (Fig. 1, Photo 16). Clearing is concentrated below Volcán Dam and along the elevated, well-drained levees. There has not yet been a systematic study of the extent or impacts associated with human-induced clearing and burning in the delta.
Water and sediment discharge through Caño Macareo (Fig. 3) has doubled as the result of dam construction, going from ~6 to 13 percent of total Orinoco River discharge. The doubling of water and sediment discharge has accelerated lateral channel migration and undoubtedly increased overbank discharge of freshwater and suspended sediment into the adjacent interdistributary basin.
It is important to keep in mind that the Orinoco Delta is, by and large, a pristine system. The broad impacts of the rather modest changes incurred by construction of the Volcán Dam serve to demonstrate the profound effects of human activities in transitional aquatic/terrestrial ecosystems without careful planning and design.
Please send comments,
questions, or suggestions to Edgar Guevara.
Copyright 1998 Bureau of Economic Geology