9.30 Streams of the Montane Humid Tropics

9.30 Streams of the Montane Humid Tropics FN Scatena, University of Pennsylvania, Philadelphia, PA, USA A Gupta, National University of Singapore, Singapore r 2013 Elsevier Inc. All rights reserved.

9.30.1 9.30.1.1 9.30.1.2 9.30.1.3 9.30.1.4 9.30.1.5 9.30.1.6 9.30.2 9.30.2.1 9.30.2.2 9.30.2.3 9.30.3 9.30.3.1 9.30.3.2 9.30.4 9.30.4.1 9.30.4.2 9.30.4.3 9.30.4.4 9.30.4.5 9.30.5 9.30.5.1 9.30.5.2 9.30.5.3 9.30.6 References

Introduction Historic Perspective Environmental Settings of TMSs Tectonic Settings Modern Climate Paleoclimate Vegetation of Tropical Montane Watersheds Hydrology and Aquatic Ecology of TMSs Runoff Generation in TMSs Floods and Storm Flows Aquatic Ecology of Tropical Rivers Water Quality and Denudation Water Quality Denudation Channel Morphology of TMSs Drainage Networks of TMSs Longitudinal Profiles and Hydraulic Geometry Channel Features Floodplains and Riparian Zones Role of Instream Wood Response to Anthropogenic Disturbances Land-Use Change Dams and Water Diversions Climate Change Conclusions

595 596 596 597 597 598 599 599 599 600 600 601 601 601 602 602 602 602 603 604 605 605 605 605 605 606

Abstract Tropical montane streams produce a disproportionately large amount of the sediment and carbon that reaches coastal regions and have often been considered to be distinct fluvial systems. They typically drain orogenic terrains that have not been recently glaciated, but have undergone climatic changes throughout the Pleistocene and currently receive 2000–3000 mm or more of precipitation each year. Steep gradient reaches with numerous boulders, rapids, and waterfalls that alternate with lower gradient reaches flowing over weathered rock or a thin veneer of coarse alluvium characterize these streams. Although their morphology and hydrology have distinctive characteristics, they do not appear to have diagnostic landforms that can be solely attributed to their low-latitude locations. Whereas they are relatively understudied, an emerging view is that their distinctiveness results from a combination of high rates of chemical and physical weathering and a high frequency of significant geomorphic events rather than the absolute magnitudes of individual floods or other geomorphic processes. Their bedrock reaches and abundance of large and relatively immobile boulders combined with their ability to transport finer-grained sediment also suggest that the restorative processes in these systems may be less responsive than in other fluvial systems.

9.30.1

Introduction

Tropical landscapes have played an important role in the scientific development of geomorphology and in evaluating the

Scatena, F.N., Gupta, A., 2013. Streams of the montane humid tropics. In: Shroder, J. (Editor in Chief), Wohl, E. (Ed.), Treatise on Geomorphology. Academic Press, San Diego, CA, vol. 9, Fluvial Geomorphology, pp. 595–611.

Treatise on Geomorphology, Volume 9

relative roles of climate, structure, process, and time in landscape development. Understanding the fluvial geomorphology of tropical streams in general and tropical montane streams (TMSs) in particular is essential, as they produce a disproportionately large amount of the sediment, carbon, and material that reaches coastal regions (Milliman and Syvitski, 1992; Lyons et al., 2002; Meade, 2007; Goldsmith et al., 2008). TMSs are also important water sources that drain some of the planet’s most diverse ecosystems and areas that are

http://dx.doi.org/10.1016/B978-0-12-374739-6.00256-6

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Streams of the Montane Humid Tropics

considered especially sensitive to environmental and climate change (Emanuel et al., 1985; Pepin and Lundquist, 2008; Colwell et al., 2008). Despite the importance of TMSs, their fundamental properties have received only limited systematic description (Wohl, 2006). This chapter reviews the current state of knowledge of the fluvial geomorphology of streams in montane tropical environments, starting with a historical perspective and their environmental settings. This is followed by a review of their morphology and their known responses to disturbances. The chapter concludes with discussion of knowledge gaps and research needs.

9.30.1.1

Historic Perspective

Studies of early tropical geomorphology were dominated by the observations of short-term visitors from temperate latitudes. This early work was inherently descriptive and typically focused on attention-grabbing features such as inselbergs (see Wirthmann, 2000; Thomas, 2006). In general, tropical landscapes were considered unique assemblages of landforms developed from long periods of intense chemical weathering in climatically and tectonically stable areas. Thus, many tropical landscapes were considered to be the end products of a Davisian-type of landscape evolution. The highly productive and diverse forests that covered these landscapes were recognized as the unique end products of millions of years of relatively undisturbed evolution. The rivers that drain these landscapes have also been considered unique since Alexander von Humboldt described the bedrock rapids of the Orinoco lowlands (Wirthmann, 2000). Subsequently, many writers have opined that the combination of intense chemical weathering, forceful rainfalls, and assumed climatic and tectonic stability has caused tropical rivers to incise and produce unique assemblages of bedrock-lined rapids and low-gradient reaches that flow over weathered bedrock covered by a thin layer of boulders and alluvium (see Wirthmann (2000) for an excellent review).

Until the mid-1950s most researchers in tropical geomorphology were based in Europe and the research focused on defining climatic–landform assemblages in cratonic settings (Budel, 1982; Kesel, 1985). In these studies, tropical mountain valleys were commonly described as narrow and V-shaped. Lowland rivers were thought to exhibit little lateral erosion and were expected to be dominated by incision (Kesel, 1985). In the past 50 years, tropical geomorphology has shifted away from its historic fixation on steady change and the hills and plains of the Gondwana continent and toward an explicit recognition of the dynamic and diverse nature of the tropics and the acknowledgment that landscapes are sculpted by a range of formative events and the restorative processes between these events (Wolman and Gerson, 1978; Scatena, 1995; Brunsden, 1996). Although inselbergs and planation surfaces are still in vogue (Coltorto et al., 2007), a much larger emphasis is currently focused on (1) quantifying the role of tropical rivers in global biogeochemical budgets (Milliman and Syvitski, 1992; Douglas and Guyot, 2004; Carey et al., 2005; Meade, 2007; Goldsmith et al., 2008) and (2) evaluating the relative roles of climate and tectonics in weathering and landscape evolution (White et al., 1998; White and Blum, 1995; Riebe et al., 2001; Hsieh and Knuepfer, 2001; Whipple, 2004; Latrubesse, 2006). Rivers of the humid tropical mountains play a central role in these debates.

9.30.1.2

Environmental Settings of TMSs

This chapter is focused on streams in mountainous regions of the humid tropics that currently receive 2000–3000 mm or more of precipitation each year. Figure 1 contains a generalized map of their occurrence that was developed from our knowledge of their distribution and by identifying ecoregions of tropical and subtropical humid montane forests (Olson et al., 2001). Nevertheless, identifying TMSs can be just as challenging and as conceptually useful as defining large rivers (Potter, 1978; Miall, 2006). As described in detail later, the TMSs considered here are located in forested montane areas

30° North

N W

30° South

0

1250 2500

E S

Kilometers 5000

Zones of tropical montane streams Isothermality ≥ 50% Major tracks of cyclones

Figure 1 The distribution of environments with tropical montane streams, paths of major cyclones, and areas where isothermality is Z50%. Isothermality is defined as mean diurnal air temperature range/monthly air temperature range and reflects tropical and subtropical climates. See text for details.

Streams of the Montane Humid Tropics

below the alpine tree line and in areas that have cooler temperatures and higher rainfall than adjacent lowland regions. They also tend to drain spatially diverse and complex geology before they enter the lowlands and coastal plain. For example, in Central America, TMS streams flowing over Pleistocene glacial deposits can be a few kilometers away from streams draining highly weathered oxisols and carbonate platforms.

9.30.1.3

Tectonic Settings

In general, TMS streams drain continental and insular mountains in active subduction zones, collision belts, rift zones, and volcanic arcs. Transform faults have also been important in the development of TMSs in the Caribbean, Taiwan, and Indonesia. They also have diverse tectonic histories and ages that have influenced their climate and geomorphic development in complex ways (Thomas, 2006). India has traversed the equator through much of the Cenozoic to form the Himalayan collision belt and the Western Ghat escarpment that are now drained by TMSs. This collision also resulted in a series of strike-slip faults that traverse Southeast Asia and influenced the location of many low mountain and hill streams that are currently drained by TMSs. Tropical South America has TMSs associated with Pacific and Caribbean–South American plate boundary interactions that date from the Cretaceous. Africa has slowly moved northward with TMS draining rift zones and uplifted sedimentary rocks, whereas Australia has moved from a near-polar position to its current subtropical location and has TMS draining Paleozoic bedrock. This diversity in geologic and tectonic history contradicts the historic notion of the old tropical Earth and indicates that many tropical landscapes and TMSs have not developed under fixed climatically or latitudinally defined conditions.

9.30.1.4

Modern Climate

Superimposed upon the geologic and tectonic variability of TMSs is a diverse set of climatic conditions. The tropics can be defined as the area of surplus radiative energy that is bounded by anticyclonic circulations near the 301 north and south latitudes (Reynolds, 1985; McGregor and Nieuwolt, 1998; Callaghan and Bonell, 2004). The climate of TMSs lacks very cold seasons and they have a consistent diurnal range of air temperature throughout the year (Hijmans et al., 2005). They also have spatially complex patterns of precipitation that result from the interaction of low-level circulation patterns, cyclonic circulation, easterly waves, and the seasonal march of the Intertropical Convergence Zone (ITCZ). Locally, precipitation patterns are also influenced by land–sea breezes, orographic uplifts, and the trade wind inversions. In many tropical mountains, these processes interact such that the zone of maximum annual rainfall occurs between 1000 and 1500 m asl (McGregor and Nieuwolt, 1998) and within the catchments of TMSs. The climates of these watersheds are typically within the Af and Am groups in the Ko¨ppen–Geiger climate classification system. In the Holdridge Life Zone system, these areas are within the lower montane to upper

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montane altitudinal belts of the tropical and subtropical moist, wet, and rain-forest life zones (Holdridge, 1967). The major climatic feature that distinguishes the humid tropics from the dry tropics is that average annual rainfall is greater than potential evapotranspiration and there is enough precipitation to support evergreen or semi-deciduous forests. Many, if not most, TMS streams drain areas that receive an annual precipitation greater than 2000 or 3000 mm yr 1. Because of the considerable seasonal variations in precipitation and runoff, it is also common in the geomorphic literature to explicitly acknowledge the presence of the seasonal and aseasonal humid tropics (Gupta, 1975, 1988, 1995). The seasonal humid tropics can be broadly defined as areas that have a marked seasonal concentration of rainfall and runoff. These areas are typically influenced by the ITCZ or the monsoonal rains and it is not uncommon that 80% of their annual stream flow occurs in 4 or 5 months of the year. Whereas most areas in the aseasonal humid tropics have a mean annual rainfall between 2000 and 4000 mm yr 1, the mean annual rainfall of the seasonal tropics is more variable and ranges between 1000 and 6000 mm. In these areas, the interannual variability of runoff is large (Mahe et al., 2004) and stream channel geometry can change dramatically between wet and dry seasons (Gupta, 1995). Interannual- to millennial-scale variability in rainfall, flooding, drought and hurricane intensity, sediment transport and deposition, water quality, and the structure of aquatic populations of TMSs have all been related, albeit complexly in many cases, to changes in sea-surface temperatures, the El Nin˜o–Southern Oscillation (ENSO), monsoons, and other global-scale circulation systems (see Douglas et al., 1999; Rodbell et al., 1999; Giannini et al., 2001; Aalto et al., 2003; Donnelly and Woodruff, 2007). In general, the interannual variability of rainfall that influences TMSs increases with decreasing rainfall, decreasing latitude, and the influence of the ITCZ and ENSO (Dewar and Wallis, 1999). A general impression exists, and in some places a misconception, that the humid tropics are characterized by a domain of steady but low-intensity rains, whereas the seasonal tropics have a higher frequency of intense rainfalls. Although inverse relationships between rainfall intensity and total rainfall have been shown in some tropical areas, these relationships are not universal (Yu, 1995). Frequent landscape-altering rainfalls are characteristic of the watersheds of TMSs and can occur in both dry and wet seasons. Multiday rainfalls over 2000 mm are not uncommon (Table 1), rainfalls greater than 500 mm d 1 typically have recurrence intervals of 20 years or less, and daily totals greater than 75 mm d 1 occur in most years (Gupta, 1988; Scatena et al., 2004; Chu et al., 2009). Rainfall intensities and event totals are commonly an order of magnitude higher in the humid tropics compared to humid temperate regions and rainfalls with intensities of 25 mm h 1 or more can account for more than 30% of annual rainfall, whereas they typically account for less than 5% in the temperate areas (Bonell, 2004). Maximum rainfall intensities also tend to occur in tropical highlands below 1500 m asl (McGregor and Nieuwolt, 1998), an elevation that is drained by TMSs. Hurricanes and cyclonic depressions traverse many TMSs but are rare in Africa, South America, and most of Southeast

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Streams of the Montane Humid Tropics

Table 1 Select examples of extreme rainfall events that have influences on tropical montane streams Total rainfall (mm per event)

Average mm d

Hurricanes 5678 3240 2467 2287 2025 1825 1524 1248 1168

1

during event

Location

Dates

Source

568 1080 1233 327 405 1825 762 1248 1168

La Reunion La Reunion La Reunion Jamaica Cuba La Reunion Jamaica Taiwan Philippines

18–27 Jan 1980 24–27 Jan 1980 8–10 Apr 1958 4–11 Nov 1909 3–8 Oct 1963 7–8 Jan 1966 5–7 Oct 1963 10–11 Sept 1963 14–15 July 1911

Landsea et al., Landsea et al., Landsea et al., Gupta, 1988 Gupta, 1988 Landsea et al., Gupta, 1988 Gupta, 1988 Gupta, 1988

Monsoon 3388 3213 1036

484 536 1036

India India India

9–16 June 1876 24–30 June 1932 14 June 1876

Gupta, 1988 Gupta, 1988 Gupta, 1988

Frontal systems and the ITCZ 2789 1109 911 867

930 1109 304 867

Jamaica Jamaica Venezuela Cuba

22–25 Jan 1960 23 Jan 1960 14–16 Dec 1999 1 June 1996

Gupta, 1988 Gupta, 1988 Planos Gutie´rrez, 2003 Planos Gutie´rrez, 2003

1999 1999 1999

1999

Source: Modified from Scatena, F.N., Planos-Gutierrez, E., Schellekens, J., 2004. Impacts of natural disturbances on the hydrology of tropical forests. In: Bonell, M., Bruijnzeel, L.A. (Eds.), Forest, Water and People in the Humid Tropics. International Hydrology Series. Cambridge University Press, Cambridge, ch. 19, pp. 489–513.

Asia (Figure 1). In regions where they are common, an average of 5–25 cyclonic storms can occur each year (Scatena et al., 2004). Where hurricanes pass directly over a TM watershed, defoliation, landslides, and flooding are widespread and can alter local and regional-scale hydrologic and nutrient cycles (Scatena and Lugo, 1995; McDowell et al., 1996; Schaffer et al., 2000; Gupta, 2000; Lyons et al., 2002; Carey et al., 2005; Goldsmith et al., 2008). Although event rainfalls can exceed 2000 mm, average rainfalls within 222 km of the eye of a hurricane are on the order of 100 mm d 1 (Anthes, 1982; McGregor and Nieuwolt, 1998). Locally their impact is strongly influenced by their storm tracks and physiography and topography (Gupta, 1988). Likewise, not all the intense rainfalls or peak discharges of TMSs are associated with hurricanes or cyclonic depressions. Multiday rainfall events associated with annual monsoons or the seasonal passage of the ITCZ may actually generate more geomorphic work and have a larger overall influence on fluvial landscapes than cyclonic systems that have recurrence intervals on the orders of decades in any location. One characteristic that appears to be relatively common in TMSs is the basin-wide nature of the intense storms. Moreover, monsoons, hurricanes, and ITZC-related storms tend to cover such large regions that even large drainage basins receive geomorphic significant rainfalls at the same time. This results in large parts of the basin contributing to and experiencing channel-modifying discharges at the same time. This is in contrast to the spatially restricted contributions of snowmelt or convective storms that often cause important but relatively localized geomorphic impacts in temperate montane streams (see Chapter 9.27). Prolonged droughts and fires are an important, but commonly underestimated, disturbance in humid tropical forests and in TMSs (Walsh and Newbery, 1999; Grau, 2001;

Malmer et al., 2004; Sherman et al., 2008). Fires are less common than droughts and the highest fire frequencies occur below the cloud forest zone and where annual precipitation is seasonal and/or less than 1000 mm yr 1. In most humid tropical forests, cumulative rainfall deficits between 5% and 10% of mean annual precipitation are common on annual and decadal timescales (Scatena et al., 2004). During droughts with recurrence intervals approaching a decade, riffles in headwater TMSs can dry up, pools can be isolated and reduced in volume, and there can be localized crowding of benthic invertebrates (Covich et al., 1998, 2003, 2006).

9.30.1.5

Paleoclimate

Geomorphic legacies of past climates are widely recognized in the fluvial environments of the midlatitudes. By contrast, because the tropics were historically considered as being climatically stable, an explicit consideration of the geomorphic legacies of past climates has not been a tradition in tropical studies. It is now known that many tropical landscapes have undergone considerable climatic changes during the Quaternary. Prior to about 28 000 14C year BP, the TMSs of Africa, South America, and Australia had experienced humid forested conditions for approximately 104 years (Thomas, 2003). During the Last Glacial Maximum (LGM), between 21 000 and 18 000 14C year BP, large parts of the tropics were cooler, rainfall was reduced by 30–60%, and there was a reduction in the extent of humid tropical forests (Servant et al., 1993; Mahe et al., 2004; Kale et al., 2003; Goodbred, 2003; Thomas, 2003 and references therein). As the glaciers retreated, rainfall and the extent of humid tropical forests increased and major changes in fluvial activity apparently took place in several tropical basins.

Streams of the Montane Humid Tropics

The early Holocene in many TMSs was relatively wet. In some areas, precipitation may have been elevated 20–35% above recent means, and 40–80% greater than the LGM minima (Goodbred, 2003; Thomas, 2003; Mahe et al., 2004 and references therein). Consequently, early Holocene changes in the fluvial environments were probably rapid and erratic and there is some evidence that many tropical rivers excavated deep rocky channels as a result of high discharges that occurred during the Pleistocene–Holocene transition. The maximum extension of modern tropical rainforests is thought to have occurred in the early Holocene (i.e., 9500–8500 14C year BP). This was followed by a mid-Holocene dry period that created favorable conditions for forest contraction, forest fires, and cut-and-fill episodes in alluvial reaches of lowland tropical streams. Humid tropical forests in Africa, Asia, Amazonia, Central America, and the Caribbean have all experienced fires and extended droughts during the past 10 000 years (Sanford et al., 1985; Hodel et al., 1991; Guilderson et al., 1994). Variations in the Holocene frequency of hurricanes and catastrophic rainfalls have also been linked to terrace formation and channel incision in Taiwan (Hsieh and Knuepfer, 2001) and coastal processes on Caribbean islands (Donnelly and Woodruff, 2007). Comparisons of the present hydrological and climatic setting with the climate regime necessary to maintain full lake levels at steady state can be used to gauge the degree to which tropical drainages are adjusted to their present climatic regime (Burrough and Thomas, 2009). This disequilibrium lake basin index suggests that equatorial lakes are currently closer to their full lake conditions at steady state than lakes in the subtropics. A similar spatial pattern of channel disequilibrium may apply to TMSs and their lowland counterparts. In any case, there is ample evidence to suggest that many TMSs have fluvial features related to Pleistocene climates and many may still be adjusting to previous climatic regimes.

9.30.1.6

Vegetation of Tropical Montane Watersheds

Process-based classifications of tropical forests are farther advanced than classifications of tropical fluvial systems. Distinctions between forests are typically based on phenology (i.e., evergreen or deciduous), climate (i.e., rain, wet, moist, or dry forests), physiography (i.e., upland and lowland), and hydrologic conditions (i.e., cloud forests, riparian, and wetland). The natural vegetation that typically covers the watersheds of the TMSs discussed here are evergreen rain, wet, moist, or cloud forests. On large equatorial mountains, the transition from montane forests to subalpine forests or grasslands is generally observed at elevations between 2800 and 3200 m (Bruijnzeel, 2001). As such, this type of land cover is only encountered in TMSs that drain the highest mountains, most of which occur in Latin America, the Himalayas, and Papua New Guinea. Currently, most watersheds drained by TMS are covered by mixtures of cut-over forests, pasture, coffee plantations, cropland, and small communities. Historically, the steep and rugged terrain of TMSs provided them with some basic level of protection. However, by the early 1990s tropical montane

599

forests were high on the list of the world’s most threatened ecosystems and they were being deforested at a rate that was considerably greater than that of lowland tropical forests (1.1% yr 1 vs. 0.8% yr 1; Doumenge et al., 1995). The total potential area of tropical montane forests has been estimated by various methods to be between 3 and 5 million km2 (Bruijnzeel et al., 2010). Approximately 45–56% of these forests remain and are drained by TMSs. Although protecting these forests is still considered a critical conservation need, the recognition of their importance as biodiversity centers and water sources has resulted in some increased legal protection. Abandonment and subsequent reforestation of watersheds drained by TMSs is also occurring in some areas (Aide and Grau, 2004). Although the impact of this reforestation on TMS morphology is uncertain, hydrologic analysis suggests that reforestation can decrease sediment yields, low-flow discharges, annual runoff, and the proportion of rainfall that contributes to stream flow (Bruijnzeel, 2004; Wu et al., 2007; Bruijnzeel et al., 2010). The influence of reforestation on peak storm flow discharge and stream power is less certain but may be proportionately less than the influence of reforestation on water quality and low stream flows.

9.30.2 9.30.2.1

Hydrology and Aquatic Ecology of TMSs Runoff Generation in TMSs

It is commonly assumed that humid tropical landscapes have quick hydrologic response times and high runoff coefficients that result in flashy streams with high peak discharges that ultimately incise channels. In practice, infiltration rates can range from 0 to over 200 mm h 1 (Harden and Scruggs, 2003) and runoff generation is complex and dependent on land cover, antecedent conditions, bedrock lithology, and basin and riparian morphology (Walsh, 1980; McDowell et al., 1992; Dykes and Thornes, 2000; Elsenbeer, 2001; Schellekens et al., 2004; Bonell, 2004; Niedzialek and Ogden, 2005; Saunders et al., 2006 and references therein). Multivariate analysis has been used to determine the relative influence of physical characteristics and land cover on the hydrology and water quality of several watersheds that contain TMSs (SantosRoma´n et al., 2003; Rivera-Ramirez et al., 2002; Soldner et al., 2004; Harmon et al., 2009). The factors most commonly linked to runoff quantity and water quality are bedrock geology, dominant land cover (i.e., forest, agriculture, and urban), and elevation, which is typically a cross-correlated surrogate of precipitation and/or land use. The most characteristic feature of the response of TMSs to precipitation is the rapid and extremely flashy nature of catchment runoff that is attributed to a variety of shallow subsurface flow paths. United States Department of Agriculture (USDA) Soil Conservation curve numbers (CNs) calculated for 28 storms in the predominantly forested Rio Chagres Basin of Panama ranged from 64 to 98 and depended on storm intensity and antecedent conditions (Calvo et al., 2005). These authors recommend a CN of 75 for extreme storms in the wet season. The area-weighted CN for TMSs and their associated lowlands reaches of northeastern Puerto Rico

600

Streams of the Montane Humid Tropics

decreased from 74 to 60.7 as barren agricultural lands and pastures were reforested (Wu et al., 2007). However, stream flow response times and the ratio of runoff to rainfall can vary within and between seasons and for some areas these figures can be relatively high at the end of the dry season when the soils are cracked and macro-pores are abundant (Niedzialek and Ogden, 2005).

9.30.2.2

Floods and Storm Flows

Flooding is common throughout the tropics and several distinct flood regimes have previously been distinguished and include: (1) occasional short-term flood; (2) frequent or annual short-term flooding; (3) annual long-term flooding; and (4) annual submersion by floodwaters (Salo et al., 1986; Scatena et al., 2004). Only the first two of these regimes are common in TMSs and TMSs in both seasonal and nonseasonal environments and are characterized by a regime where short-term events capable of transporting bedload (see Chapters 9.8 and 9.27) and removing periphyton (see Chapter 9.12) occur several times each year. Peak discharges are several orders of magnitude larger than base flow but average annual peak flows are generally not channel-forming or modifying events, especially in areas with boulders and bedrock-lined channels (Scatena et al., 2004; Pike et al., 2010; see Chapter 9.28). Major channel-modifying events have been associated with peak discharges that range from 20.9 to 65 m3 s 1 km 2 and have recurrence intervals on the order of decades (Gupta, 1988; O’Connor and Costa, 2004; Garcin et al., 2005). The defoliation and uprooting associated with hurricanes can also produce considerable amounts of nutrient-rich green litter and wood (see Chapter 9.11) that can clog channels and even cause temporary reductions in suspended sediment yields (Lodge et al., 1991; Gellis, 1993; see Chapter 9.9). Although the importance of flooding to TMSs is widely acknowledged, so is the difficulty in estimating the recurrence intervals of moderate to extreme floods in ungauged TMSs (Pike and Scatena, 2010). In a well-gauged TMS network in Puerto Rico, flood discharges that are close to the annual peak are commonly experienced several times in a year. Comparative analysis of these streams also showed that in these flashy and relatively small streams, annual maximum flow series analysis fails to capture the intra-annual flows that are responsible for structuring the vegetation in and adjacent to the channels. A partial duration series based on 15-min discharges is recommended for most analyses.

9.30.2.3

Aquatic Ecology of Tropical Rivers

Research on the aquatic ecology of tropical rivers has highlighted differences between tropical and temperate streams (see Chapter 9.12), including the latitudinal variations in diversity, radiation, temperature, geostrophic effects, and the influences of continuous litter inputs, warm water, the lack of ice, and common high flows (Payne, 1986; Jackson and Sweeney, 1995; Talling and Lemoalle, 1998; Dudgeon, 2008). Recent efforts have focused on the spatial variability in ecological processes in relation to waterfalls and other

geomorphic conditions that influence within-channel habitats and the migration and distribution of species (Wantzen et al., 2006; Boyero et al., 2009). Although studies of tropical stream metabolism that extend for at least 1 year and/or extend along the longitudinal profile of a basin are scarce, available information suggests that rates of in stream photosynthesis in forested TMSs are similar to those of similarly sized streams draining temperatedeciduous forests (Ortiz-Zayas et al., 2005). However, continual herbivory and a high frequency of bedload-transporting storms interact to suppress the abundance of periphyton and submerged aquatic plants in TMSs. Consequently, their rates of respiration are much higher than in most temperate streams and they can have ratios of photosynthesis to respiration of less than 1 from their headwaters to their lower reaches. Nevertheless, where tropical rainforest vegetation is present, it can provide streams with sufficient amounts of labile organic carbon to support high rates of respiration over long distances and make tropical streams globally important sources of carbon inputs to oceans (Kao and Liu, 1996; Lyons et al., 2002; Ortiz-Zayas et al., 2005). Short-term floods and droughts cause significant invertebrate mortality and shifts in population-age distributions in TMS streams in Malaysia, Hong-Kong, India, Ecuador, tropical Australia, the Andean piedmont of Venezuela, and the Caribbean (Flechter and Feifarek, 1994; Scatena et al., 2004). However, native populations have numerous morphological and behavioral adaptations to deal with common, substratedisturbing stream flows, including suction-cup-like appendages that can cling to bedrock surfaces and the ability to hide under large boulders or occupy shallow-water channel-margin habitats during floods and droughts. Some pan-tropical shrimp and fish species can migrate past vertical waterfalls that are tens of meters high by migrating in bedrock joints and in areas with moss coverings and laminar sheet flow. Although frequent floods and steep gradients characterize TMSs many, if not the majority, of the native macro fauna living in TMSs migrate between rivers and coastal zones over the course of their lives (March et al., 1998, 2003; March and Pringle, 2003; Crook et al., 2009). Consequently, waterfalls, dams, and other geomorphic or anthropogenic migration barriers can have important roles in determining the community composition and longitudinal variation of aquatic species in TMSs. Coastal conditions, the distribution of waterfalls and migration barriers, altitude, drainage area, riparian and watershed land cover, water quality, substrate size, and pool volume have all been correlated to the abundance of aquatic organisms and the structure of aquatic communities in TMSs (Pyron et al., 1999; Fievet et al., 2001; Zimmerman and Covich, 2003; Soldner et al., 2004; Blanco and Scatena, 2005, 2006). Because of the importance of floods, waterfalls, and other geomorphic barriers to the distribution of aquatic organisms in TMSs, the question has been raised whether the river continuum concept (RCC), which successfully explains longitudinal patterns in species distributions and food webs in temperate streams (Vannote et al., 1980), also applies to TMSs. In general, the RCC suggests that the longitudinal distributions of aquatic species reflect downstream changes in channel morphology, discharge, sediment load, riparian cover,

Streams of the Montane Humid Tropics

and incident radiation on the water surface. Although geomorphic barriers do cause discontinuities in the distribution of organisms in TMSs the few existing controlled studies suggest that the general patterns of food web and resource availability that are predicted by the RCC do exist (March and Pringle, 2003; Greathouse and Pringle, 2006). It has also been hypothesized that the ecology of TMSs might be functionally closer to montane temperate streams than to their lowland counterparts and that lowland tropical streams might differ substantially from lowland temperate streams (Boyero et al., 2009). Increased knowledge of the life histories and habitat requirements of tropical species are desperately needed to quantify and understand longitudinal patterns and differences between TMSs and temperate streams.

9.30.3 9.30.3.1

Water Quality and Denudation Water Quality

TMS streams have warmer water, higher annual exports of dissolved constituents and sediment, and less seasonal differences in water temperature and water chemistry than their temperate counterparts. Their water quality is not influenced by freeze–thaw cycles, ice-induced bank erosion, or seasonal pulses of plant litter (although hurricane deforestation can create significant pulses). Several studies have related stream water concentrations of certain elements to either shallow near-surface flow or longer and deeper flow paths (Schellekens et al., 2004; Bonell, 2004; Bhatt and McDowell, 2007; Saunders et al., 2006). Surface soils drained by TMSs are typically wet, commonly saturated, and have intensive microbial activity that can reduce and remove nitrogen from soil and groundwater before it enters the stream channel (Chestnut et al., 1999; Chestnut and McDowell, 2000). Local riparian nitrogen dynamics and their ability to remove nitrogen before it enters the stream channel depend on local lithology and geomorphology (McDowell et al., 1992) and some of the planet’s highest basin-average rates of denitrification are found in the humid tropic systems in South America and Africa (Seitzinger et al., 2006). Most carbon exports from TMSs are in the form of dissolved organic carbon (McDowell and Asbury, 1994; Chestnut et al., 1999; Lyons et al., 2002). Land-use change and largescale hurricane-related defoliation can result in significant increases in stream water cation and carbon exports. However, post-hurricane exports are less than a few percentages of the hurricane-derived plant litter inputs, which reflects the tight nutrient retention these systems can have (Schaffer et al., 2000). Nevertheless, because of their high carbon exports and storm-initiated CO2 consumption from silicate weathering, some TMSs subject to tropical cyclones may be important global sinks of CO2 transport to ocean burial (Kao and Liu, 1996; Lyons et al., 2002; Goldsmith et al., 2008; Draut et al., 2009).

9.30.3.2

Denudation

The average rate of ground surface lowering of TMSs can be well over 100 m per million years, but typically ranges

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between 50 and 75 m per million years (White et al., 1998; Hsieh and Knuepfer, 2001; Hartshorn et al., 2002; Thomas, 2003; Riebe et al., 2004; Whipple, 2004 and references therein). The sediment in most TMSs is ultimately derived from weathered saprolite, which typically contains meterdiameter core stone boulders in a matrix of clays, silts, and sands. Saprolite thickness varies widely and ranges from a few meters to more than 100 m and comparisons of long-term denudation rates with the rate of saprolite advance suggest that the saprolites drained by some TMSs have reached their steady-state thickness (see Buss et al., 2008 and references therein). Rates of chemical denudation in TMSs are some of the largest in the world (Milliman and Syvitski, 1992; White and Blum, 1995; Syvitski and Milliman, 2007) and TMSs underlain by granite or on volcanic islands where meteorologic waters are impacted by high subsurface temperatures are among the highest of TMSs (Brown et al., 1995; Riebe et al., 2001; Rad et al., 2007). The ratio of physical denudation to total denudation in the drainages of TMSs is variable and there are insufficient studies for a definitive analysis. Nevertheless, available rates suggest that physical denudation for TMS drainages can range between 40% and 75% of total denudation and averages around 60% (White et al., 1998; Riebe et al., 2001; Buss et al., 2008; Harmon et al., 2009 and references therein). Slope failures tend to contribute most of the river sediment in TMS, irrespective of the scale of the basin. In some areas, landslides producing rain storms occur on an average of once every 1–2 years and rainfall intensity-duration curves indicate that slope failures can occur with most types of tropical rain events, including the annual migration of the ITCZ, hurricanes, convective storms, and cold fronts (Scatena et al., 2004). Available comparisons suggest that the rainfall thresholds needed to trigger slope failures may be higher in the humid tropics than in temperate areas (Larsen and Simon, 1993; Gabet et al., 2004). However, because the frequency of these rains is higher, landslides have a significant influence on hillslopes and channel processes. The most common slope failures are shallow translational failures that have depths less than 10 m (Simon et al., 1990; Larsen and Torres Sa´nchez, 1992; Maharaj, 1993; Paolini et al., 2005). Whereas most slope failures are moisture driven and occur in the wet season or following large tropical storms, earthquake-generated landslides can also be significant in many TMS drainages (Garwood et al., 1979). It is also common in TMSs that large flows can transport years to decades worth of average annual sediment and material flux in a single event. Modeling studies on the humid tropics of Australia indicate that 30% of the total rainfall contributes approximately 87% of the total transported sediment but only 45% of the total runoff (Yu, 1995). In Puerto Rican streams, the highest recorded daily sediment discharges are 1–3.6 times the annual suspended-sediment discharge, and runoff from major storms transport 1–32 times the median annual sediment load (Warne et al., 2005). In eastern Jamaica, stream flows associated with Hurricane Gilbert transported large quantities of coarse bed sediment and over 1700 times the daily suspended load of the dry season (Thomas, 1991; Gupta, 2000). In Taiwan, individual typhoons

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can have hyperpycnal sediment concentrations (440 g l 1) and can transport between 72% and 95% of the annual particulate organic carbon fluxes of the highest yielding world rivers (Goldsmith et al., 2008). Studies in Taiwan also suggest that valley lowering and channel incision are driven by relatively frequent flows of low to moderate intensity, whereas large and rare floods are more important in widening bedrock channels (Hartshorn et al., 2002). The highest sediment yields in the humid tropics also originate from TMSs in tectonically active regions (Douglas and Guyot, 2004). Whereas the global-scale battle regarding who is responsible for delivering the most fluvial sediment to the ocean may be fought between geology, geography, and humans (Syvitski and Milliman, 2007), TMSs underlain by highly weatherable bedrock and in areas subject to hurricanes and occupied by marginalized farmers appear to be the winners. Moreover, soil erosion in anthropogenically disturbed TM watersheds can be several orders of magnitude larger than pre-disturbed rates (Anderson and Spencer, 1991; Douglas et al., 1992; Hewawasam et al., 2003; Douglas and Guyot, 2004; Sidle et al., 2004; Warne et al., 2005). These studies suggest that TMSs draining undisturbed forested watersheds typically have sediment yields around 100–500 t km 2 yr 1. Large, mixed land-use watersheds can have sediment yields between 1000 and 3000 t km 2 yr 1. Small, highly disturbed areas associated with logging can have yields greater than 25 000 t km 2 yr 1 (Sidle et al., 2004).

9.30.4 9.30.4.1

Channel Morphology of TMSs Drainage Networks of TMSs

The hillslopes that drain into TMSs are typically characterized by landslide scars and a dense network of intermittent swales and channels that dissect the landscape into narrow interfluves and deep valleys. Reported drainage densities of TMSs range from 2.6 to over 20 km/km 2 (Scatena and Lugo, 1995; Walsh, 1996; Terry, 1999). In general, drainage densities in humid tropical areas are considered to be higher than in humid temperate areas because of higher precipitation intensities but lower than in semiarid areas because of greater vegetation cover (Chorley et al., 1984). Detailed analyses of several tropical areas further suggest that the relationship between drainage density and annual rainfall in TMS networks is nonlinear and influenced by extreme daily rainfall totals and the permeability, mineralogy, and storage capacity of soils (Walsh, 1996). Analysis of several TMS networks indicates that drainage density increases relatively rapidly until approximately 2500–3000 mm yr 1, at which point it increases at a reduced rate with further increases in annual rainfall (Walsh, 1996). These studies also indicate that these networks commonly failed to conform to Horton’s laws of stream numbers and that although the high channel densities can develop in less than 50 years, major changes in basin or network shape do not occur before 14 000 years. The drainage networks of TMSs are commonly described as rectangular and structurally controlled and as having straight nonaccordant tributaries that join the main channels at high angles (Ahmad et al., 1993; Hare and Gardner, 1985;

Ng, 2006). On the Nicoya Peninsula of Costa Rica, drainage basin asymmetry has been used to identify centers of uplift and direction of tilt (Hare and Gardner, 1985). In the Greater Antilles, the rectangular network morphology of TMSs has been related to strike-slip, plate-boundary tectonics (Ahmad et al., 1993), and in Hong Kong the headwater progression of the drainage network has been related to systematic variations on landslide morphology and density (Ng, 2006). It is typically unclear whether the slope breaks and steps at the junctions of a tributary and the mainstem are actively retreating knick-points, structural, or high-flow features. These nonaccordant tributary junctions have been known to influence the upstream migration of aquatic species.

9.30.4.2

Longitudinal Profiles and Hydraulic Geometry

Average stream gradients of TMSs are typically well above the 0.002 m m 1 threshold that has typically been used to define montane streams (Wohl and Merritt, 2005, 2008). Their longitudinal profiles are typically described as being segmented by waterfalls and alternating steep and lower gradient segments with morphology correlated to bedrock morphology. For example, along the upper Rio Chagres watershed of Panama, reaches flowing across granites, diorites, and tonalites have lower gradients and wider channels than reaches flowing across gabbros and diorites (Wohl, 2005). In the Luquillo Mountains of Puerto Rico, lower stream gradients are associated with granodiorite, whereas steep gradients are associated with more erosion-resistant contact metamorphic rocks (Pike et al., 2010). In Fiji and Hawaii, waterfalls can also occur where stream flows across more resistant lava flows (Terry, 1999). Hydraulic geometries of many mountain streams are highly variable and complex (Wohl and Merritt, 2005; see Chapters 9.18 and 9.27). Nevertheless, the few available studies suggest that TMSs may have better developed downstream hydraulic relationships than their temperate counterparts, especially where compared to montane streams in temperate areas that have been recently glaciated (Pike et al., 2010). Unfortunately, the interpretation of hydraulic geometry in mountain streams in general, and TMSs in particular, is complicated because of the lack of identifiable bankfull or reference discharges to compare channel geometry at constant flow frequencies. Because of the lack of welldefined floodplains or other bank-full features, most studies of the hydraulic geometry of montane streams have based hydraulic geometries on reference discharges that are associated with recent floods (Pike et al., 2010). Nevertheless, in TMS reaches of Puerto Rico that lack a floodplain, a definable channel boundary that is characterized by the incipient presence of soil, woody shrubs, and trees corresponds to the same flow frequency as the bankfull discharge of nearby alluvial channels. The reference discharge based on these riparian features has an average exceedance probability between 0.09% and 0.30%, and a recurrence interval between 40 and 90 days.

9.30.4.3

Channel Features

Bedrock channels (see Chapter 9.28), boulder bars and boulderlined channels, step–pools (see Chapter 9.20), pool–riffle

Streams of the Montane Humid Tropics

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sequences (see Chapter 9.21), and all of the morphologic features observed in other mountain streams (Montgomery and Buffington, 1997; Thompson et al., 2006) have all been observed in TMSs. Although it is generally considered that the abundance of bedrock channels or bedrock–alluvial channels (sensu Whipple, 2004) is relatively high in TMSs, a comprehensive data set does not exist to verify this quantitatively. Nevertheless, reaches with a continuous or deep cover of alluvial sediments are generally lacking, whereas reaches with accumulations of large-diameter boulders as well as boulder leeves, boulder bars, and boulder steps are common (Figures 2 and 3). The origin of the boulders varies, as some large boulders are exhumed core stones whereas others have been transported to channels during slope failures and debris flows (Ahmad et al., 1993; Terry, 1999). Globally, TMSs in the following areas are considered to have the intense rainfalls, steep slopes, and the geologic substrate that produce coarse-grained material to maintain the morphology created during large floods (Gupta, 1988): (1) river valleys of East Asia, especially Taiwan and the Philippines; (2) upland areas of Vietnam, Sumatra, Java, and Burma; (3) humid areas of the Indian subcontinent; (4) Madagascar and neighboring parts of coastal East Africa; (5) North and Northeast Australia; and (6) Caribbean basin and highlands of Central America. In some regions, the morphology of tropical stream channels has also been related to a pronounced seasonality in stream flow. Streams in the seasonal dry Kimberley Plateau of tropical Australia have a unique channel system where narrow bedrock-lined reaches alternate with wider alluvial reaches that have sandy ridges and anabranching channels (Wende and Nanson, 1998). In areas with large storms and large seasonal fluctuations in discharge, alluvial reaches of TMSs can have a nested morphology that consists of a large storm flow channel and a smaller channel that carries interstorm discharges (Gupta, 1995). The interstorm channels are box shaped and have high banks and high width–depth ratios. The high-magnitude floods can occupy the entire valley bottom and are sufficiently frequent that the high-flow

channels features are maintained. This type of multiple lowand high-flow channels appears to be most common and pronounced in areas subject to monsoon rains.

Figure 2 The Rio Mameyes River in the Luquillo Mountains of Puerto Rico.

Figure 3 Headwater tropical montane stream in the Luquillo Mountains of Puerto Rico.

9.30.4.4

Floodplains and Riparian Zones

Continuous alluvial floodplains or riparian zones (see Chapter 9.14) are rare in TMSs. Instead, channel margins are most commonly boulder-lined, steeply sloping hillslopes, or consist of smaller patches of alluvium associated with tributary junctions, slope breaks, or former slope failures. Floodplains and associated terraces are more common in middle and lower reaches of the drainages. In Fiji, cesium profiles in floodplain sediments indicate accretion rates of 3.2 cm yr 1 over the past 45 years (Terry et al., 2002). These high accretion rates are attributed to the high frequency of tropical cyclones that pass the area (40 between 1970 and 2002). Although alluvial riparian zones along TMSs are discontinuous and typically occupy less than 10–15% of the landscape, they can be significant sources of storm flow (Schellekens et al., 2004) and have large influences on light, temperature, and the carbon and nitrogen chemistry of TMS water (Chestnut and McDowell, 2000; Chestnut et al., 1999; Heartsill-Scalley and Aide, 2003; MacKenzie, 2008). In the subtropical wet forests of the Luquillo Mountains in Puerto Rico, steep topographic and hydraulic gradients between the

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riparian zone and the stream were responsible for nearly constant inputs of groundwater to these first- to third-order streams (McDowell et al., 1992, 1996). However, the abundance and hydrologic and biogeochemical influence of riparian zones were highly dependent on geology such that areas underlain by granodiorite that weather into deep sandy soils have stronger hydrologic and biogeochemical connection than areas underlain by volcanic rocks that weather into clay. These floodplains tend to be smaller and are characterized by surface drainage and periodically dry surface soils. The floodplains of larger lowland tropical streams offer striking and well-documented examples of how the frequency and duration of flooding and floodplain soil saturation are linked to patterns of forest structure and biodiversity (Salo et al., 1986; Kalliola et al., 1991; Mertes et al. 1995; Hamilton et al., 2007 and others). Relationships between fluvial processes and riparian vegetation have also been documented in a few TMSs. Riparian zones along the first- to third-order TMSs of the Luquillo Mountains of Puerto Rico do not have distinct riparian species or riparian communities but do have distinct understory species (Heartsill-Scalley et al., 2009). Relationships between the flood frequency and the structure of riparian vegetation have been documented in these streams (Pike and Scatena, 2010). The width of their riparian zones defined on the basis of canopy cover, understory vegetation, and soil drainage averages 22 m for perennial channels and 10 m for intermittent channels (Scatena, 1990). For comparison, timber harvesting guidelines for Australian tropical forests require leaving a minimum strip of undisturbed forests of 10 m for streams draining less that 60 ha and 20 m for channels draining 100 ha or more. No buffer protection is required where channels are less than 5m wide. In Peninsular Malaysia, the amount of logging-derived sediment reaching the channel declined after 40 m but the overall effectiveness of riparian buffers depends on the hydrologic connectivity between hillslope and channel buffers (Gomi et al., 2006). In summary, these studies indicate that TMSs can have distinct zones of riparian vegetation that are on the order of 10–40 m wide. For comparison, in the relatively flat terra firme landscape of the Central Amazon, riparian zones defined by a distinct, high-diversity riparian herb community can be 100 m wide (Drucker et al., 2008).

9.30.4.5

Role of Instream Wood

Logjams and accumulations of coarse woody debris (CWD) are known to play an important role in structuring the morphology and habitat in forested temperate streams (see Chapter 9.11). The few studies that have investigated CWD in tropical streams indicate that although TMSs lack beavers and other large river dwellers, debris packs created by CWD, palm fronds, and fine litter do provide important habitat and food resources to the detrital-based aquatic food webs of TMSs (Covich and Crowl, 1990). However, CWD appears to be less abundant in TMSs than in some temperate counterparts. A detailed survey of 26 montane to lowland stream reaches in the Dominican Republic indicated that 62% had measurable CWD, but no reach has more than 5% woody debris cover (Soldner et al., 2004).

In first-order TMSs in a pasture–forest landscape mosaic in Puerto Rico, the amount of CWD tended to increase with forest cover and there were positive relationships between tree cover and percentage of dissolved oxygen, and negative relationships between tree cover and percentage of substrata covered by fine-grained sediments from eroded soil (Heartsill-Scalley and Aide, 2003). A 4-month CWD addition experiment in pools in headwater TMS streams also indicated that the CWD additions were correlated to changes in aquatic species composition but had no effect on the total number of freshwater shrimp per pool area (Pyron et al., 1999). A study of the transport of numbered, 2-cm-diameter hardwood dowels in a second-order, boulder-lined stream in the Luquillo Mountains indicated the dowels are dispersed in a negative exponential pattern and have high retention at the reach scale, even during large, hurricane-related storm flows (Covich and Crowl, 1990). This high retention is attributed to CWD, palm fronds, and other plant material being entangled in crevices between boulders. It has also been noted that suspended sediment concentrations during hurricanes can be lower than predicted from concentration–discharge relationships derived from nonhurricane storms of similar magnitudes (Gellis, 1993). Apparently, defoliation by the hurricane-force winds created temporary debris dams that trapped sediment and reduced suspended sediment concentrations. Nevertheless, because relatively high stream flow can persist for several days, the total sediment transported during the passage of a hurricane can be significant (Gupta, 2000; Warne et al., 2005). Unlike some cold temperate streams where CWD dams can last for decades or centuries, CWD dams in TMSs are removed on the order of years. In the Malaysian State of Sabah, CWD dams have an average life span of approximately 1 year, although some can exist over 10 years (Spencer et al., 1990). In the Upper Rio Chagres Basin of Panama, large CWD dams produced by a widespread flooding and landsliding event lasted 2 years or less and the fluvial system appears to alternate between brief periods with moderate CWD loads and long periods with minor CWD inputs (Wohl et al., 2009). Although a few of the CWD dams that are produced by hurricane defoliation and uprooting in the Luquillo Mountains of Puerto Rico last as long as 5 years, the majority of CWD dams in headwater streams were broken and redistributed within less than 6 months and CWD dams have not been observed in third- and fourth-order streams. The long-term average rate of CWD inputs into most TMSs is unknown, but it should be similar to that observed in temperate environments because tree mortality and the rate of stand turnover are similar in tropical and temperate forests (Lugo and Scatena, 1996). However, in hurricaneimpacted areas, and in areas undergoing deforestation, the average annual rate may be larger or at least more episodic. Available decay rates of CWD tissue in warm tropical streams indicate they can decay between 16% and 30% over 3 years (Beard et al., 2005). These relatively rapid decay rates, combined with the high frequency of storm flows and episodic inputs, apparently interact to reduce the life span of CWD accumulations and their ultimate influence on channel morphology.

Streams of the Montane Humid Tropics

9.30.5

Response to Anthropogenic Disturbances

Because TMSs drain hillslopes with abundant weathering products and because they have many storms per year and high sediment loads, it could be assumed that they can recover from formative events or adjust to new environmental conditions faster than their temperate or arid counterparts. However, because many TMSs are also supply-limited with respect to fine sediments, they may not have the material needed to rapidly reform and adjust their channels in response to environmental changes. Although time will tell whether the presumed effectiveness in restorative processes of TMSs actually is true or part of the dynamic but stable mythology associated with TMSs, there is no doubt that TMSs are undergoing significant changes because of human activities.

9.30.5.1

Land-Use Change

The influence of land-use change (see Chapter 9.37) on hydrologic process and runoff of TMSs has been documented in several locations (see many examples in Bonell and Bruijnzeel (2004)). In general, the conversion of forested watersheds to pastures and cropland increases erosion, runoff, and sediment yields. Available studies indicate that PO4, K, and Mg concentrations increased considerably with urbanization and the water-quality changes associated with agriculture and urbanization in the humid tropics are of similar magnitudes and directions as temperate streams (SantosRoma´n et al., 2003; Ramirez et al., 2009). However, the longterm geomorphic response of tropical stream channels to land-cover change and urbanization (see Chapter 9.39) is poorly quantified and no studies have focused on TMSs (Douglas, 1978; Gupta, 1982, 1984, 2010; Gupta and Ahmad, 1999; Ebisemiju, 1989a, 1989b; Chin, 2006; Jeje and Ikeazota, 2002; Ramirez et al., 2009). In humid temperate environments, streams commonly aggrade during the initial phases of urbanization in response to the increased sediment loads that are generated during construction. As the urban landscape becomes established, channels then tend to enlarge in response to an increased frequency of high-flow events that carry less sediment. A recent review of the limited studies from urban tropical areas suggests that channel enlargement from tropical urbanization tends to be smaller in magnitude compared to temperate counterparts (Chin, 2006). Slight downstream decreases in channel size have also been observed in coastal plain streams in Puerto Rico and have been related to the presence of sediment deposited in earlier agricultural periods (Clark and Wilcock, 2000). By contrast, in streams draining established urban areas of Puerto Rico, the amount of channel incision does not appear to be correlated with urbanization and river connectivity seems to be more important than urbanization in determining fish assemblage composition (Ramirez et al., 2009). Unfortunately, existing studies on the impacts of urbanization on tropical streams have been short term and may be biased toward the initial stages of construction and aggradation. Thus, the long-term influence of urbanization or other land-cover changes on the morphology of TMSs is uncertain. However, given that these

605

systems have few alluvial reaches and already capable of transporting more fine sediment than is supplied, they are not expected to undergo the same pattern of aggradation and widening as alluvial reaches in humid temperate areas.

9.30.5.2

Dams and Water Diversions

Because of their high runoff and montane settings, TMSs are generally well suited for hydroelectric generation or gravitydriven water diversions (Benstead et al., 1999; Pringle et al., 2000; Brasher, 2003; March et al., 2003). In Central America, hydropower from TMSs already generates approximately 50% of the electricity and the number of dams and diversions is expected to continue to increase in the future (Anderson et al., 2006a, 2006b). From an ecological view, dams and diversions can be similar to extended droughts and result in a reduction in resident and migratory habitat and the crowding and accelerated mortality of individuals (see Chapter 9.38). The cumulative effects of these alterations can be a decrease in riffle habitats and in the number of fish species immediately downstream from the dams. The cumulative impacts of multiple hydroelectric dams releasing water at the same time each day are unclear but of concern to many residents who live downstream of TMSs.

9.30.5.3

Climate Change

As noted earlier, the climate of most TMSs has changed in the past and will change in the future (see Chapter 9.40). The general expectation is that in the next century tropical mountains will undergo warming and drying that will result in an upward shift in life zones (Colwell et al., 2008). Local and upwind deforestation can also influence precipitation patterns in the watersheds of TMSs (Bruijnzeel et al., 2010). The relatively high atmospheric inputs of nutrients and efficient internal nutrient cycles suggest that the biogeochemical systems of tropical montane forests will rapidly adjust to future environmental changes (Bruijnzeel et al., 2010). How and when the morphology of TMSs will respond to climatic and environmental change is less certain. However, if the future resembles our admittedly poor understanding of the past that is discussed above, widespread drying should result in cut-andfill episodes in alluvial reaches in the foothills of TMSs. Increases in precipitation and forest cover should promote relatively fixed, stable channels with riparian areas covered by mature rainforest. Changes to the morphology of the steeper gradient, boulder- and bedrock-lined channels of TMSs are expected to reflect changes in the frequency of slope failures and debris flows. Changes in biogeochemical weathering rates and in the fluxes of water and sediment from TMSs are also expected and may be good geoindicators of environmental change in these systems (Osterkamp, 2002).

9.30.6

Conclusions

Although TMSs have a long history of fascinating and confusing geomorphologists, it is only recently that a rudimentary understanding of their fluvial geomorphology can be

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developed from process-based case studies. Whereas the paucity and restricted geographic distribution of available studies still limit our ability to develop rigorous predictions of their behavior, the emerging view that is summarized below can be used to guide future research and management:

•

•

•

•

Most TMSs drain orogenic terrains that have not been glaciated but have undergone climatic changes throughout the Pleistocene and Holocene. In many areas, early Holocene precipitation was 20–35% above recent means and 40–80% greater than during the drier LGM. Drying and an upward shift in life zones are expected for the future and an ongoing challenge is to identify the geomorphic legacies of these past climatic fluctuations and the response of TMSs to future changes. TMSs typically receive 2000–3000 mm yr 1 or more of precipitation and have a high frequency of intense and, in some cases, prolonged rainfalls that commonly impact the entire watershed at the same time. Rainfall and discharge can be seasonal and temporal changes in runoff coefficients are common. TMSs drain steep hillslopes with high drainage densities and shallow subsurface storm flow paths that rapidly deliver precipitation to stream channels. Their rectangular channel networks closely reflect regional geologic structure, whereas the slopes and widths of their segmented longitudinal profiles reflect underlying bedrock. High channel densities can develop in a few decades but drainage networks commonly fail to conform to Horton’s laws of stream numbers and length. TMSs have high material fluxes from both physical and chemical weathering and are the headwaters of streams that may contribute between 20% and 40% of the global fluxes of dissolved load and sediment to the oceans. Their chemical denudation is strongly influenced by deeply weathered and thick saprolite and tight internal biogeochemical cycles. Available data suggest that physical denudation averages around 60% of total denudation and the recurrence interval of landslide-generating events is on the order of years.

The net result of these interactions are storm-dominated fluvial systems that are characterized by a high frequency of short-duration events that efficiently transport dissolved material and fine sediment. Most TMS streams are considered to be supply-limited with respect to fine-grained sediment and transport-limited with respect to the large boulders that enter the channel during debris flows or by in situ weathering. The stream channels in these systems are characterized by:

•

•

Steep-gradient streams with numerous boulders, rapids, and waterfalls that alternate with low-gradient reaches flowing over weathered rock or a thin veneer of coarse alluvium. Knick-point migration, differential weathering rates, and debris flow– hillslope interactions are all responsible for the development of waterfalls and rapids in TMSs. A future challenge will be to determine the relative importance of these processes in different tectonic and geologic environments. Better developed downstream hydraulic geometries than their temperate montane counterparts. This may be due to some combination of the lack of recent glaciations and because

•

•

•

the deeply weathered saprolite is relatively deformable given the high frequency of intense storms they experience. Lack of permanent CWD that structures channel and reach morphology. Although the long-term supply of CWD to TMSs may be similar or even larger than the supply to temperate montane counterparts, the combination of episodic inputs, rapid decomposition, and mechanical breakdown by a high frequency of storms apparently reduces the residence time and overall geomorphic influence of CWD. Poorly developed and discontinuous floodplains. Distinct riparian zones can be identified on the basis of soils and vegetation and typically extend 20–40 m from either side of headwater channels. Because shallow subsurface flow commonly passes through wet and nearly saturated riparian soils before entering TMS channels, the biogeochemical transformations within the riparian zone can have a disproportionate influence on stream water chemistry. Therefore, establishing riparian buffers zones can be an effective best management practice in these systems. Migratory aquatic species that are well adapted to floods and to migrating steep bedrock channels. Their spatial distribution within TMS networks is directly related to the distribution of waterfalls or anthropogenic barriers. The high frequency of bedload-transporting storms combined with continual herbivory interacts to suppress the abundance of periphyton and aquatic plants. Consequently, rates of respiration are much higher than most forested temperate streams.

As the above review suggests, TMSs do not have diagnostic landforms that can be solely attributed to their low-latitude locations. However, there is general agreement that TMSs are distinct fluvial systems. The distinctiveness of TMSs appears to result from a combination of high rates of chemical and physical weathering and a high frequency of significant geomorphic events rather than the absolute magnitudes of individual events or processes. It is generally assumed that stream channels tend toward quasi-equilibrium morphologies because their beds and profiles deform and adjust in response to changes in discharge and sediment supply. In many TMSs, described relationships between bedrock lithology, channel slope, and channel morphology suggest that these channels do adjust and establish quasi-equilibrium morphologies. However, the abundance of large and relatively immobile boulders and their lack of fine-grained alluvial deposits suggest that the restorative processes in these systems may be less responsive than those where fine-grained sediment is actively involved in rebuilding and sculpting channels between formative events. A future challenge in understanding and managing these systems under ever-increasing anthropogenic pressures is to distinguish the formative and restorative events that sculpt these landscapes and maintain their aquatic resources and contributions to regional biogeochemical cycles.

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Biographical Sketch F.N. Scatena has worked (and played) in tropical montane streams since 1977, when he was a US Peace Corps Volunteer hydrologist in the Dominican Republic. After receiving his PhD from Johns Hopkins University, he spent 16 years as a research hydrologist at the USDA International Institute of Tropical Forestry in Puerto Rico. Since 2001 he has been a professor of earth and environmental science at the University of Pennsylvania. His publications include two edited volumes on Tropical Montane Cloud Forests and over 140 peer-reviewed journal articles on topics ranging from ecosystem management to the geomorphic and biogeochemical influences of hurricanes.

Avijit Gupta was educated at Presidency College, Calcutta and the Johns Hopkins University, Baltimore. He has taught in various universities in India, USA, Singapore, and UK. His current research involves rivers impacted by large floods and seasonality, large rivers, remote sensing, and effect of climate change on rivers and deltas. His publications include about 70 papers and 9 books including the edited volume, Large Rivers, and about to be published, Tropical Geomorphology.