Episodic wood loading in a mountainous neotropical watershed

Geomorphology 111 (2009) 149–159

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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h

Episodic wood loading in a mountainous neotropical watershed Ellen Wohl a,⁎, Fred L. Ogden b, Jaime Goode a a b

Department of Geosciences, Colorado State University, Fort Collins, CO 80523-1482, USA Department of Civil and Architectural Engineering, University of Wyoming, 1000 E. University Avenue, Laramie, WY 82071, USA

a r t i c l e

i n f o

Article history: Received 24 October 2008 Received in revised form 14 April 2009 Accepted 16 April 2009 Available online 23 April 2009 Keywords: Panama Flood Mountain river Large woody debris Logjam Tropics

a b s t r a c t The Upper Rio Chagres drains 414 km2 of steep, mountainous terrain in central Panama. A tropical air mass thunderstorm on 10 July 2007 produced a flood across the basin that peaked at 720 m3 s− 1 at a headwaters gage draining 17.5 km2 and 1710 m3 s− 1 at a downstream gage draining 414 km2. The storm also triggered numerous landslides in the upper basin, which facilitated the formation of large logjams along portions of the channel where transport capacity of wood was reduced by a change in channel geometry such as a bend or channel expansion. During field work in February 2008, we characterized three jams with surface areas of 400–2450 m2; two of these jams resulted in storage of substantial (1100–8200 m3) sediment wedges upstream. We returned to these sites in March 2009 to document changes in the logjams and sediment storage. Drawing on observations made in the basin since 2002, and site visits during 2008 and 2009, we suggest that jams such as these last two years or less. We propose that wood dynamics in the Upper Chagres alternate between brief periods of moderate wood load in the form of large logjams and much longer periods of essentially no wood load, a situation that contrasts with the more consistent wood loads in catchments of similar size in temperate environments and with limited studies of more consistent wood load in tropical catchments with no landslides. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Relatively small, mountainous rivers exert a strong influence on local-, regional-, and global-scale hydrology, geomorphology, and aquatic ecology (Milliman and Syvitski,1992; Freeman et al., 2007), yet less is known about the basic physical and ecological processes and forms of these river systems than about larger, lowland river systems (Wohl, 2000). Knowledge of tropical mountain rivers is particularly limited by the relative dearth of field studies conducted in these environments thus far. The absence of field data makes it difficult to compare such basic attributes as rates and magnitude of change and temporal and spatial variations in process and form between tropical catchments and mountain rivers in other regions. Research on the geomorphic and ecological influences of wood in mountain rivers, for example, has grown exponentially during the past decade to encompass case studies from a wide variety of geographic settings (Meleason et al., 2005; Wyzga and Zawiejska, 2005; Comiti et al., 2006; Mao et al., 2008), yet almost nothing on this subject has been published for the tropics. Although tropical mountainous catchments will likely exhibit as wide a variety of hydrologic and geomorphic processes as temperate mountainous catchments, the presence of extremely intense rainfall

⁎ Corresponding author. Tel.: +1 970 491 5298. E-mail address: [email protected] (E. Wohl). 0169-555X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2009.04.013

(Gupta, 1975; Gilmour and Bonell, 1979; Scatena, 1995), rapid development of deep weathering profiles (Lam, 1978), and rapid biological decay of wood (Delaney et al., 1998; Clark et al., 2002) might produce consistent differences in tropical mountain rivers relative to other mountain rivers. This possibility can be examined as more field studies are conducted across a range of tropical environments. We contribute to the growing literature on tropical mountain rivers by examining the hydrologic and geomorphic processes associated with a moderate flood that occurred in July 2007 in the Upper Rio Chagres, a mountainous catchment of central Panama. The Upper Rio Chagres above the Chico gaging station (Fig.1) drains 414 km2 of steep terrain in the seasonal tropical rainforest of Panama. Extensive field work in the basin during 2002 revealed a lack of large wood in the stream channels, despite the mature rainforest covering the catchment. Intense rainfall over the entire basin in late December 2000 produced a flood that peaked at 2004 m3 s− 1, which has an estimated recurrence interval of 20 years based on statistical analysis of 36 years of annual peak flows (1972–2007). Although recent landslide scars were observed in the catchment in 2002, none of these appeared to be directly influencing channels at that time (i.e., no debris fans constricting channels or continuing sediment input to channels). A storm on 10 July 2007 that was centered over the headwaters of the Upper Chagres dramatically changed this situation, triggering numerous landslides that reached the channel and introduced sufficient wood to form large jams at several sites in the upper catchment. Our primary objectives in this paper are to document the hydrology and

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Fig. 1. Location of the Upper Rio Chagres basin, showing the location of HEC-RAS modeling reaches (⁎HR) and other localities mentioned in the text. Jam 1 is immediately upstream from Upper Chagres base camp, and jam 2 and the headwaters gage site are immediately downstream from the base camp. Jam 3 coincides with the HEC-RAS modeling reach at the junction of the Chagres and Chagrecito.

geomorphic effects of this flood and to use our observations from 2002 and 2007–2009 to develop a conceptual model of episodic geomorphic change along this mountainous neotropical stream network. We start with a discussion of measured and inferred precipitation and discharge characteristics, and then document the geomorphic effects of sediment and wood introduced to the channel via landslides. 2. Field area The Rio Chagres drains 3340 km2 in central Panama. The construction of Madden Dam in 1934 divided the watershed into upper and lower catchments separated by the extensive Lake Alhajuela. Our field area is in the Upper Chagres (Fig. 1), which drains 414 km2 of steep, mountainous terrain. The 60-km-long Upper Chagres descends 800 m from its headwaters at 890 m elevation to its base level at 88 m in Lake Alhajuela. Hillslopes exceed 45° in over 90% of the Upper Chagres catchment (Larsen, 1984; Robinson, 1985). Extensive small landslides along these steep slopes, along with frequent tree fall, generate locally uneven slope topography (Harrison et al., 2005). Bedrock geology is a mixture of volcanic and intrusive rocks of Cretaceous to Upper Tertiary age that are highly deformed and chemically altered (Wörner et al., 2005). Andesitic dikes that range in thickness from 1 to 3 m cross-cut all lithologies (Wörner et al., 2005). Bedrock outcrops discontinuously along the channels throughout the basin and is seldom more than a few meters below the channel alluvium. The majority of the Upper Chagres lies within the Parque Nacional Chagres, although 30% of the land is in private ownership. Land use is primarily cultivation that is restricted to these private land holdings, and road and trail access is minimal. Land cover in the Upper Rio Chagres basin is old-growth forest (98%), pasture (1%) and grassland (1%) (Ibáñez et al., 2002). No botanical studies have been conducted within the riparian zone in the Upper Chagres basin, but upland inventories near our study sites indicated a mean of 155 tree species ≥1 cm in diameter within four 40× 40 m plots, and a total of 435 tree species

(Pérez et al., 2005). Trees can attain a height of 30 m and a diameter of 2.2 m (R. Condit, Smithsonian Tropical Research Institute, pers. comm., 2003), although many trees have a diameter ≤1 m. Mean annual precipitation is not measured at the Chagres headwaters, but recorded data at Lake Alhajuela indicate a mean annual precipitation of 2590 mm during 1914–1995. Approximately 90% of this rain falls during the wet season of May–December (Houseal, 1999). A strong precipitation gradient exists across the isthmus of Panama, with maximum precipitation near the Caribbean coast, decreasing towards the Pacific coast. Mean annual precipitation in headwater regions nearer to the Caribbean is likely to greatly exceed that at Alhajuela (Knox et al., 2005). Preliminary soils investigations suggest that soil water repellency causes a disproportionate increase in runoff during the first few storms after the dry season (Calvo et al., 2005; Niedzialek and Ogden, 2005). Subsurface runoff dominates hillslopes during the wet season, with infrequent positive soil water pressure leading to non-Darcian flow through soil pipes and macropores during intense rainfall (Hendrickx et al., 2005). Discharge in the Upper Rio Chagres has been measured at the Chico gage, a few km upstream from Lake Alhajuela, since 1933. Discharge typically varies between 10 and 20 m3 s− 1 during the dry season, with multiple flood peaks over 1000 m3 s− 1 during the wet season. Mean annual peak flow is 1049 m3 s− 1, and the flood of record was 3780 m3 s− 1 in November 1966. This equates to a peak discharge per unit area of 9 m3 s− 1 km− 2, although much higher unit discharges have been recorded in subbasins (e.g., 41 m3 s− 1 km− 2 in July 2007). Discrete quasistable base flows are present in the Upper Rio Chagres, but at least some tributaries exhibit more ephemeral behavior throughout the year (Niedzialek and Ogden, 2005). During multiple dry- and wet-season visits between 2002 and 2009, turbid flows (indicating storm runoff) were frequently seen on one or two tributaries to the Rio Chagres. Most tributaries remain clear, indicating that flows in the river typically result solely from base flow. The response time of the catchments is quite fast. Water-level loggers deployed at six subcatchments within the watershed starting

E. Wohl et al. / Geomorphology 111 (2009) 149–159

in 2005, combined with analysis of weather radar imagery provided by the Autoridad del Canal de Panama, indicate that at scales near 20 km2 the response time from centroid of rainfall to centroid of the runoff hydrograph is ~1.25 h. This represents a theoretical average flow velocity of 1 m s− 1, which is quite high and supports the notion that non-Darcian quick interflow processes are very important. The exact mechanisms of runoff production remain to be identified, but the data clearly show that these tropical catchments covered by oldgrowth forest are very responsive to rainfall. Downstream hydraulic geometry is well developed in the Upper Chagres despite local variability in channel morphology associated with changes in bedrock lithology and exposure (Wohl, 2005). Headwater step-pool channels give way to pool-riffle morphology farther downstream and in lower gradient segments of the watershed. The relative paucity of large wood in the channels and of recent landslides that reached the channels was noticeable during a month of field work in 2002. 3. Methods Our field work focused on the portion of the Rio Chagres between the Upper Chagres base camp and the Rio Piedras junction (Fig. 1). Precipitation during the July 2007 storm was focused on the Upper Chagres above the Chagrecito junction, and helicopter overflights along the Chagres at the start of field work in February 2008 revealed that landslides along the valley walls and wood deposited along the channel occurred predominantly upstream from the Piedras confluence. We visited the sites discussed in this paper in February 2008 and March 2009, allowing us to observe changes through time in the logjams and associated stored sediments.

151

Table 1 Locations of direct or indirect discharge estimation. Location

Drainage area (km2)

Method of discharge determination

Headwaters gage

18

720

Chagres–Chagrecito reach Chagres–Piedras reach Chico

60

700

Pressure-transducer water-level logger HEC-RAS modeling

270

1200

HEC-RAS modeling

407

1749

Gage operated by the Panama Canal Authority

default values of contraction and expansion coefficients], the stepbackwater method iteratively calculates an energy-balanced watersurface elevation between successive surveyed cross sections. The elevations of high flow indicators at cross sections along the surveyed reach are then compared to the computed water-surface elevations at the respective cross sections. The water-surface profile is adjusted by varying discharge until the computed water-surface profile best matches the field-surveyed, high flow indicators. High flow indicators in the Upper Chagres modeling reaches included organic debris, scour lines in hillslope colluvium, and vegetation damaged by the flood. 3.2. Geomorphology We measured intermediate diameter of 100 clasts along high flow bars at the HEC-RAS modeling sites using the random walk method (Wolman, 1954). We used these data to estimate the mobility of coarse bed sediments with the selective entrainment criterion of Komar (1987):

3.1. Hydrology and hydraulics

0:6

0:4

τi = 0:045ðρs − ρÞgD50 Di The Autoridad del Canal de Panama (ACP) maintains a network of seven recording rain gages in and around the Upper Rio Chagres watershed. This is not a particularly high density of rain gage coverage for the watershed. To augment rainfall observations, the ACP operates a 10-cm-wavelength weather radar on a hilltop near Panama City. This radar collects data that are combined with rain gage data for bias adjustment, providing near real-time rainfall estimates. The ACP has also provided runoff data from a gaging station on the Rio Chagres just downstream from the confluence with the Rio Chico (referred to here as the Chico gage). This data set represents the longest continuous record in existence from a forested old-growth watershed of its size. Beginning in 2004, we installed nonvented pressure-transducer water-level loggers at six locations within the Upper Rio Chagres basin, representing subcatchments draining areas ranging from 1 km2 to 80 km2. One of these sensors is installed at a location in the headwaters of the Rio Chagres at a scale of 17.5 km2. This gaging site is referred to as the “headwaters” gage. We surveyed two channel reaches intermediate in distance downstream between the headwaters and Chico gage sites for the purpose of indirect discharge estimation (Table 1). At each site, we surveyed five channel cross sections that we used in the step-backwater model HECRAS (HEC, 1995). This model calculates a one-dimensional watersurface profile that depends on discharge, channel roughness, and channel geometry (O'Connor and Webb, 1988; HEC, 1995). We modeled flow as subcritical, which requires estimation of watersurface elevation at the downstream-most cross section based on high flow indicators surveyed in the field. We treated flow as subcritical because of the large boundary roughness associated with vegetated islands that were submerged during the 2007 flood, the consistent channel width and gradient in the reaches chosen for modeling, and the lack of any abrupt changes in the elevation of high flow indicators within the reach. For a specified discharge and assumed friction and form energy losses [n values based on Barnes (1967) and HEC-RAS

July 2007 flood Q (m3/s)

ð1Þ

where ρs is sediment density (2.65 g/cm3), ρ is water density (1 g/ cm3), g is gravity (9.8 cm/s2), D50 is the median of the intermediate axis measurements for all clasts at a site, and Di is the intermediate diameter of the clast for which τi is being determined (here D84). We surveyed three large logjams between the Upper Chagres base camp and the Chagres–Chagrecito junction that were located at sites accessible by helicopter or by walking from helicopter landing sites. At each jam, we surveyed (i) the outer margins of wood accumulation so that we could calculate surface area of the jam, (ii) the volume of sediment stored upstream from the jam, if upstream storage was present; (iii) the length and diameter of the largest pieces of wood incorporated into the jam; and (iv) channel dimensions upstream and downstream from the jam so that we could infer changes in transport capacity of wood during the July 2007 flood. We calculated the volume of sediment stored from a wedge shape based on jam thickness, channel bed gradient, and probable upstream extent of the wedge as indicated by changes in grain size and bed gradient. We assessed the potential mobility of wood at each of the jam sites using three measures. (i) The ratio of piece length of wood to channel width (Llog/w) reflects the greater potential for wood to move as this ratio drops below 1 (Swanson et al., 1984; Lienkaemper and Swanson, 1987). (ii) The ratio of flow depth to log diameter (dpeak/Dlog) reflects the greater potential for wood to move as flow depth exceeds half of the log diameter (Abbe et al., 1993). (iii) Wood is likely to move via floating (rather than rolling) when the floating index h⁎ of Bocchiola et al. (2008) exceeds a threshold of 1.26,   ⁎ ⁎ h = Dlog ρw = ρlog

ð2Þ

where D⁎log = dpeak / Dlog, ρw is water density (1 g/cm3) and ρlog is the log density. We do not have data on wood density for trees at the Chagres study sites, but Clark et al. (1999) documented a range of

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Table 2 Result of HEC-RAS modeling and characteristics of study reaches.a Reach

Qp (m3/s)

S (m/m)

v (m/s)

τ (N/m2)b

Ω (W)

ω (W/m2)1

D50, D84 (cm)

τi for D84 (N/m2)

Chagres–Chagrecito Chagres–Piedras

700 1200

0.005 0.002

2.8, 1.5 4.8, 3.0

38, 25 294, 185

34,300 23,520

114, 50 1488, 720

19, 33 20, 40

17 19

a b

Qp is flood peak discharge, S is channel gradient, v is mean velocity, τ is mean shear stress, Ω is mean total stream power, and ω is mean stream power per unit area. First value is average within active, low flow channel; second value is average within high flow channel.

values of 0.35–0.98 g/cm3 for tropical rain forest trees in Costa Rica. Assuming that the density of waterlogged trees is slightly higher, we used a range of 0.4–1.1 g/cm3 in our calculations of h⁎. 4. Results 4.1. Hydrology and hydraulics of the July 2007 flood The 10 July 2007 event was a localized, intense, tropical air mass thunderstorm. Radar observations show that nearly continuous heavy rainfall commenced over the entire Upper Chagres basin at approximately 14:00 UTC (Coordinated Universal Time) and ended by 17:00. The nearest ACP rain gage, which is located approximately 3.9 km west–southwest of the headwaters gage, recorded no rainfall for the 10 July 2007 event. Malfunction is suspected because no rainfall was recorded at this gage between 8 and 24 July, an unusual occurrence. The next nearest ACP rain gage is located in the headwaters of the Rio Esparanza, approximately 8.4 km northwest of the headwaters streamflow gage at an elevation of 552 m. The Esparanza rain gage recorded rainfall beginning from 12:45 UTC, and ending after 17:15 UTC, with 14, 15-minute periods of rain in excess of 30 mm h− 1 and two 15-minute periods of rainfall in excess of 100 mm h− 1. The measured storm-total rainfall at the Esparanza rain gage for this event was 235 mm, which is likely an underestimate due to well

known problems with tipping bucket rain gages in heavy rain. Runoff recorded at the headwaters gage commenced at 14:24, reached a peak of 720 m3 s− 1 at 16:20, and ended at 19:11 UTC. This is the largest peak discharge recorded at the headwaters gage since monitoring began in 2005, and represents a specific peak discharge of 41 m3 s− 1 km− 2 at the scale of 17.5 km2. At the Chico gage downstream, the 10 July flood peaked at 1710 m3 s− 1. This was the second highest peak recorded during 2007; the peak for the year at the Chico gage was 1780 m3 s− 1 on 26 November, an event that produced a modest 168 m3 s− 1 peak discharge at the headwaters gage. Modeled peak discharge and average values of hydraulic variables for each of the two HEC-RAS reaches are summarized in Table 2. At the Chagres–Chagrecito reach, flood stage was 6–7 m above base flow water's edge. A discharge of 700 m3 s− 1, with n values of 0.03 in the main channel and 0.07 in overbank areas, produced a water-surface profile that matched highwater marks surveyed in the field. Modeled average velocity within the main channel was moderately high (2.8 m/s), although the broad, vegetated left margin of the channel and associated large hydraulic roughness produced much lower average velocity for the entire flood channel (1.5 m/s). Flood stage was 3.5–4.5 m above base flow water's edge at the Chagres–Piedras reach. A discharge of 1200 m3 s− 1, with n values of 0.045 in the main channel and 0.05–0.07 in overbank areas, produced a good fit between computed water-surface profile and surveyed highwater marks.

Table 3 Jam characteristics. Surface area of jama (m2)

Piece L, db (m)

Drainage areac (km2)

Jam 1

2400; 300

16

Jam 2

400; 310

Jam 3

2450; 600

27.2, 0.53 21.0, 0.35 14.9, 0.34 23.9, 0.80 19.2, 0.43 3.9, 1.90 18.4, 0.72 15.1, 0.50 15.4, 0.80 19.0, 0.45 16.5, 0.58 10.2, 0.28 6.8, 0.20 3.6, 0.20 12.8, 0.25 15.5, 0.27 8.3, 0.37 14.8, 0.33 11.8, 0.25 10.5, 0.27

Location

a

Wood ratiosd Llog/w, dpeak/Dlog

h⁎e

80

0.21, 2.1

6, 2

20

14h

0.97, 8.6

25, 9

60

128

0.11, 25

71, 26

AF (m2)

Channel ratiosf w

d

S

Sediment volume (m3)g

0.41

0.9

0.15

8200; 5100

–

1100; 600

0.21

–

0.29

1.0

First number is surface area in 2008, second number is surface area in 2009. L (length) and d (diameter); these dimensions are for the largest key pieces in each jam; average values are given in bold. c Upstream drainage area on main channel. d Ratios are based on average wood and channel dimensions. Values of channel width and flow depth are for mainstem (not tributary, in the case of jam 2). e Values of the floating index are for the lower and upper bounds of assumed ρlog, respectively, and average values of other variables. f Channel ratios are for cross section upstream and downstream from each jam. This measure was not relevant for jam 2, which is at the mouth of a tributary channel. g Estimated volume of sediment stored upstream and downstream from jam 1 and upstream from jam 2, based on channel geometry and jam thickness. No sediment storage was associated with jam 3. First number is for 2008, second number is for 2009. h This frontal area is for the front of the jam with respect to tributary flow. b

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4.2. Geomorphic effects of the July 2007 flood The peak flows modeled in HEC-RAS suggest that the bed material at both the Chagres–Chagrecito and Chagres–Piedras reaches was mobile during the July 2007 flood (Table 2). During February 2008 field work we noted that the bed material at all the sites we visited above the Chagrecito junction was slightly more angular and contained more fine (silt and clay) sediment than in 2002, suggesting that sediment contributed by landslides continued to affect bed material characteristics 7 months after the flood. Comparison of values for D50 and D84 at sites that were visited in 2002 and again in 2008, however, indicated negligible change in these parameters. Characteristics of the three logjams are summarized in Table 3. Jam 1 is located in a bend where the channel widens dramatically at the head of a forested island (Fig. 2A). Jam 2 is located at the mouth of a tributary channel that enters about a kilometer downstream from jam 1, where the mainstem is a narrowly constricted bedrock gorge (Fig. 3A). Another jam (jam 2′) is located at a bedrock constriction a few hundred meters upstream along the tributary (Fig. 3E). Jam 3

153

formed immediately downstream from a bend and the junction of the Rio Chagrecito, where the channel widens substantially at the head of a forested island (Fig. 4). Comparison of channel parameters at an upstream site representing average channel dimensions and at a site immediately downstream from the jam for jams 1 and 3 indicates that main channel width and gradient increase and flow depth remains relatively constant (Table 3). Because discharge is constant between each pair of cross sections, this suggests a decrease in velocity as flow enters the reach where the jam formed. The presence of vegetated islands at these sites also presumably reflects decreased velocity during peak flows, facilitating bedload deposition and stabilization by woody vegetation. The increased overbank roughness associated with each island would further reduce velocity and facilitate wood retention. Each of the jams was substantially reduced in size when we returned to the sites in March 2009 (Table 3). Dimensionless ratios Llog/w and dpeak/Dlog reflect the high mobility of wood in the Upper Chagres. Only in the constricted bedrock gorge where jam 2 is located does wood length begin to approach channel width (Table 3). The Llog/w values qualify the Upper

Fig. 2. (A) View downstream to jam 1 in February 2008; person at right for scale. Forested island downstream from jam appears to be right bank in this view; right bank lies behind the first row of trees and is not visible here. (B) View across jam 1 to downstream bar in March 2009. The upper surface and downstream face of the bar are more dissected than in 2008. (C) Aerial view of jam 1 in March 2009. Flow is from the upper right to the bottom of the view in the main channel; a secondary channel is out of the view at left. Only 3 large logs still span the main channel.

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Fig. 3. (A) View across the main channel (flow right to left) to the mouth of the tributary and jam 2 in February 2008. Very large piece extending across main channel in foreground is 15.4 m long and 80 cm in diameter. Note also the person at left for scale and the small landslide scar along the tributary at upper right of photograph. (B) Equivalent view across the main channel in March 2009. The downstream end of the jam is breached and the upstream end of the jam is being covered in vegetation, as is the landslide scar, which is no longer visible at upper right. (C) View down the tributary toward the main channel in February 2008, illustrating the sediment and wood stored upstream from jam 2. Person at center for scale. (D) View up the tributary in March 2009, immediately upstream from jam 2, showing dissection of sediment wedge and fill terraces along channel margins. (E) View upstream along tributary to next jam, a few hundred meters upstream from jam 2. Person for scale at right below jam.

Chagres sites as large rivers in Gurnell et al.'s (2002) classification. The values of dpeak/Dlog for the Upper Chagres also greatly exceed theoretical ratios for wood entrainment (Braudrick and Grant,

2000); and the values of h⁎ are substantially larger than the threshold value of 1.26, suggesting that wood in the Upper Chagres floats readily.

E. Wohl et al. / Geomorphology 111 (2009) 149–159

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Fig. 4. (A) Low-elevation aerial view of the Chagres–Chagrecito HEC-RAS modeling reach, with jam 3 at the upstream end (not visible here). Photograph, taken in July 2007 from a helicopter, shows a slightly tilted view. The mainstem flows from the top toward the bottom of the view. The pale gray band at right is a secondary channel that had a cobble-boulder bed in 2002, but a sand bed in 2007. The main channel continues straight beyond the bottom of this view and then takes a sharp bend to the left; the 2007 flood waters cut across the inside of the bend and deposited the wood that appears as pale gray lines in this view. Note also the wood deposited on the vegetated island between the main and secondary channels. (Photograph courtesy of Russell Harmon.) (B) February 2008 view of the same reach. Vegetation has begun to grow back along the island between the main channel and the secondary channel and on the inside of the bend. (C) Downstream view toward part of jam 3 at the head of the secondary channel in February 2008; forested island at right in this view, and main channel farther right and out of view. Person at center for scale. (D) Downstream view in March 2009. The jam is largely gone and coarser sediment is re-exposed at the head of the secondary channel.

We also made very rough estimates of the volume of sediment stored in association with jams 1 and 2 (jam 3 did not appear to cause substantial sediment storage). Jam 1 might be associated with a large volume of coarse sediment deposition, rather than causing it. This jam is located downstream from a bend, with extensive (50 m long, 50 m wide) cobble and boulder bars immediately up- and downstream from the jam. Similar cobble and boulder bars also occur, however, in association with channel bends that do not have logjams. The estimated volume of sediment storage for jam 1 in Table 3 is thus a maximum value; actual sediment storage caused by the jam might be much lower. D50 for the bar sediment above jam 1 is 11 cm; D84 is 18 cm. When we returned to this site in March 2009, however, we found that the portion of jam 1 across the main channel was largely

gone (Fig. 2C) and the downstream bar was dissected and reduced in size (Fig. 2B), suggesting that jam 1 does influence downstream sediment storage. Jam 2 is more likely to be the cause of a wedge of sand- to gravelsized sediment stored immediately upstream, along the mouth of the tributary channel (Fig. 3C). When we returned to this site in March 2009, jam 2 was breached along its right side (downstream with respect to the main channel) (Fig. 3B). A cut approximately 3 m wide and 4 m deep had eroded to bedrock. This triggered incision of the sediment wedge upstream, steepening the tributary gradient and leaving fill terraces 1– 1.5 m tall along the channel margins (Fig. 3D). The next jam up the tributary (jam 2′) remained largely intact in March 2009, although a small breach at one side of the jam had triggered upstream incision of

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stored sediment, analogous to that above jam 2 but of lower magnitude in terms of depth and upstream extent. Although jam 3 did not create a wedge of sediment storage upstream, it may have contributed to fining in the secondary channel downstream. When we visited this site in 2002, substrate in the secondary channel was primarily cobble and boulders. In February 2008 the secondary channel was covered in sand (Fig. 4B, C) at least 20 cm thick. By March 2009 jam 3 was largely gone, the upstream portion of the secondary channel was again cobble and boulder, and the downstream portion had a thinner veneer of sand over cobble and boulder (Fig. 4D). 5. Discussion Field observations during February 2008 indicate that wood is introduced to the mainstem Upper Chagres via bank erosion (a relatively minor source), landslides along the valley walls of the main channel, and tributary inputs that include both landslides and bank erosion along the tributary. Individual landslides are not particularly large (estimated ≤270 m3), but they are sufficiently closely spaced along the mainstem of the Chagres above the Chagrecito junction (average one landslide every 250 m along the channel) that the cumulative contribution of wood and

Fig. 5. (A) View across the mainstem (flow right to left) to the largest landslide observed during the February 2008 fieldwork. This landslide is upstream from jam 2. Although coarse, angular sediment remains along the channel margin, flow has eroded the debris fan such that the channel bank is likely close to the pre-landslide location, as inferred from the large tree growing along the channel immediately downstream from the landslide. Person at bottom center for scale. (B) Similar view in March 2009. Vegetation is quickly covering the landslide scar and thus reducing continuing sediment inputs to the channel from this site.

sediment is substantial. The landslides and logjams along the tributary at jam 2 also indicate that tributaries in the upper catchment contributed substantial volumes of wood and sediment to the mainstem during the July 2007 storm. Although larger landslides and tributary inputs might temporarily dam flow on the main channel or at least partially constrict mainstem flow and cause some ponding, the absence of fans at landslide toes or tributary junctions during the February 2008 field work was striking (Fig. 5A). We interpret this to reflect the tremendous transport capacity of the mainstem Chagres during high flows, as suggested by the values of hydraulic variables estimated during HEC-RAS modeling relative to the threshold conditions for transport of bed sediment and wood. We did not see any evidence of debris flows resulting from landslides, which further supports the interpretation of tremendous fluvial transport capacity associated with rapidly rising high water discharges. The partial dissection of sediment stored upstream from logjams on the tributary that we observed in March 2009 also provides some insight into the narrow fill terraces that we observed along many tributary channels in 2002. At that time, there was no obvious mechanism for explaining these terraces, but it now appears that logjams lasting a year or two facilitate upstream sediment deposition and formation of narrow fill terraces when the logjam that temporarily formed a local base level eventually disperses and the tributary incises. Once wood reaches the mainstem, it tends to aggregate at locations of flow separation and reduced transport capacity. Jam 1 occurred at the upstream end of a large bend and zone of split flow around a forested island, jam 2 occurred at the mouth of a tributary, and jam 3 occurred immediately downstream of a tributary junction at the head of a zone of split flow around a forested island. The large number of pieces oriented roughly 30–90° to the flow on the Upper Chagres, as well as the width of the jams transverse to flow, suggest that the jams may have resulted from congested transport of wood (Braudrick et al., 1997). This makes sense in the context of large point sources of wood entering the flow from landslides. The high mobility of wood in the Upper Chagres presumably facilitates the formation of large jams when pieces stop moving. Manners et al. (2007) reported values of jam frontal area, AF, for the Indian River in New York (drainage area ~370 km2, S 1%, Qp 40 m3 s− 1, w 40 to 60 m) of 2.7, 6.3, and 14.6 m2, respectively, for 3 jams. Values for the Upper Chagres jams are much larger (Table 3), despite the similarity in drainage area and channel width between the two catchments. These observations lead to the conceptual model summarized in Fig. 6. Intense rainfall triggers landslides along the mainstem and tributary channels that introduce sediment and wood into the channel, as well as flooding in the channel network. Flow and channel geometry along the tributary control the downstream movement of wood and sediment toward the main channel, just as flow and channel geometry along the mainstem control further downstream movement of wood and sediment. Wood clusters into jams at sites of flow separation and reduced transport capacity, including tributary mouths and the upstream portion of forested islands immediately downstream from a channel bend. If the logjam is sufficiently wide and tall, it can create enough of a backwater effect to reduce bedload transport immediately upstream and promote formation of a wedge of bed material (this occurred at jams 1 and 2, but not jam 3). As landslide occurrence declines over the next few months (Fig. 5B), subsequent stream flows and biological decay of the wood cause logjams to disperse downstream, allowing the sediment wedge to also disperse. Logjams become very rare along the mainstem until the next intense rainfall triggers further landsliding. We estimate that the time for dispersal of logjams is b2 years, based on observations in 2002 and knowledge of the 2000 flood, as well as observations of logjams during 2007–2009. A smaller flood on December 28, 2007 (peak discharge 292 m3 s− 1 at headwaters gage site) had already begun to remove some of the wood deposited in jam 3 by the July 2007 flood, and this removal was largely complete by March 2009 (Fig. 4).

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Fig. 6. Schematic planview illustration of the conceptual model of logjam formation along the Upper Rio Chagres study area. Scenario I represents a time less than a year after an intense rainfall triggers landslides and flooding. Scenario II represents a time ~ 2 years after landslides and flooding.

Previous investigators have documented similar scenarios of landslides rapidly recruiting large volumes of wood into a channel, followed by the formation of logjams and storage of sediment (Swanston and Swanson, 1987; Benda et al., 2003; May and Gresswell, 2003; Nakamura and Swanson, 2003), with an increase in jam frequency up to drainage areas of ~300 km2 (Abbe and Montgomery, 2003). What makes the scenario on the Upper Rio Chagres unique is the low spatial frequency of jams even shortly after the 2007 storm, the extremely limited duration of the logjams, and the large changes in wood load along the mainstem between the two extremes of the scenario illustrated in Fig. 6. The relatively few long-term studies of wood distribution and retention along mountain streams in other environments indicate relatively little change in wood load, although individual pieces of wood are exchanged and the position of jams changes (Gurnell et al., 2002; Gurnell, 2003; Wohl and Goode, 2008). Studies in the temperate rainforest environments of the Pacific Northwest indicate mean wood residence times of 12 to 100 years in streams of various sizes, with maximum residence time exceeding 1000 years in some systems (Hassan et al., 2005). Gregory et al. (1985) and Gurnell (2003) documented jams that persisted for less than a year on very small streams (catchment b12 km2), but other investigators documented persistence of jams for a decade or longer on streams draining areas of 9–30 km2 (Haschenburger and Rice, 2004; Wohl and Goode, 2008). Retention of jams that do not completely span the channel appears to increase to timespans of multiple decades or even centuries on much larger rivers (N1000 km2) in the temperate zone (Abbe and Montgomery, 1996; Montgomery and Abbe, 2006). Previous investigators have also consistently documented the highest wood loads in the smallest channels, with decreasing wood load per unit area of channel as transport capacity increases downstream (Swanson et al., 1982; Wohl and Yaeger, 2009). The 2002 fieldwork

along the Chagres revealed almost no wood in even the steepest, smallest channels, despite evidence that treefall was common on the adjacent, steep valley walls. We found logjams in both small tributaries and the mainstem channel only after the July 2007 storm. The very limited retention of logjams and individual pieces of wood in the channels of the Upper Rio Chagres presumably reflects the combination of rapid decay and very high transport capacity during flood peaks. Living plants were colonizing the logs remaining in jams 1– 3 by March 2009, which may facilitate decay. Although investigators have not even identified woody riparian species along the Upper Chagres, we can infer something about decay rates from studies in slightly lower elevation forests of Costa Rica and at slightly lower elevations along the Panama Canal. Ecologists at La Selva Biological Station in Costa Rica (elevations 34–110 m) have found that turnover time of fallen logs on the forest floor averages 9 years (Clark et al., 2002), although fallen trees from species particularly resistant to decay can remain on the forest floor for up to 20 years (Janzen,1983). Bultman and Southwell (1976) summarized the results of a 13-year study of decay in heartwood stakes from various Panamanian tree species exposed at sites near the canal. Twenty-seven percent of the species exhibited heavy to very heavy decay within 2.5 years, and this percentage rose to 63% by 7.5 years and 80% by 13 years. Decay may be more rapid in fluvial environments with repeated wetting and drying and abrasion of wood. We also noted more individual large trees along the channel margins downstream from each jam in March 2009 than we had seen in February 2008, presumably as a result of fluvial transport and dispersal of these logs as the jams broke up. The absence of wood in even small channels during 2002 suggests that even very large individual trees are retained for a few months to, at most, a couple of years in stream channels of varying size. Numerous landslides throughout the catchment can temporarily overwhelm transport capacity,

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promoting the formation of logjams and associated sediment storage, but even the jams are a relatively transient feature. The Upper Chagres is thus supply-limited with respect to wood (Marcus et al., 2002) most of the time, even though the bankfull width is much less than the lower limit of 200–300 m proposed for this condition in temperate streams. During helicopter overflights of the Upper Chagres mainstem in 2008 and 2009, we noted that no logjams were present and very few pieces of wood were located along the channel margins downstream from the Piedras junction. This lower portion of the Upper Chagres is a nearly continuous bedrock gorge. The absence of wood might reflect (i) the fact that wood from the July 2007 storm, during which rainfall was centered over the upstream portion of the Upper Chagres, had not reached this lower portion of the mainstem more than a year after the flood; or (ii) wood trapped in jams and along channel margins of the sinuous, incised bedrock gorge in the upper portions of the Upper Chagres is abraded and decays so rapidly that only small pieces of wood (much shorter than active channel width) make it down to lower portions of the network. The latter explanation is supported by the absence of wood in the lower portion of the Upper Chagres and at the inlet to Lake Alhajuela in 2002, two years after another large storm that likely created landslides and introduced wood throughout the entire Upper Chagres. One other consideration is that downstream from the Rio Piedras junction, the transport capacity of the channel may allow transport of wood to Lake Alhajuela in the unlikely rare event that the entire Upper Chagres basin is impacted by an extreme rainfall event. River stages associated with the flood peak of record, 3780 m3 s− 1, exceeded 24 m in this portion of the Upper Chagres. The transport and fate of wood under this scenario might be different from the events we have analyzed, and might have important implications for the transport of carbon out of the watershed and subsequent burial within Lake Alhajuela. A truly extreme event focused on the lower watershed could potentially cause plugging of the Madden dam spillway by wood. The Upper Chagres also forms an interesting contrast to smaller headwater channels (drainage area 0.1–8.5 km2) in Costa Rica (Cadol et al. 2009). Although these channels are also in a region with rapid rates of wood decay and have high values of discharge per unit drainage area, they are mostly narrower channels of lower gradient and recruit wood through individual tree fall or limited bank erosion. Ongoing work indicates that wood load in 50-m-long segments of these small channels varies by less than a factor of three from year to year and that even small jams are uncommon (D. Cadol, Colorado State University, pers. comm., 5/08). We interpret the more uniform spatial and temporal wood loads of the Costa Rican streams to reflect the absence of landslides and associated large point sources of wood recruitment to the channels. 6. Conclusions Our observations of wood dynamics over a period of several years in the Upper Rio Chagres suggest that the upper basin alternates between brief periods of moderate wood load in the form of large jams triggered by accumulations of wood introduced primarily through landslides following intense rainstorms and much longer periods of essentially no wood load. The bimodal characteristics of wood load in the Upper Chagres contrast with more consistent wood loads in catchments of similar size in temperate environments and with very limited studies of more consistent wood load in smaller tropical catchments. Extreme temporal variations in wood load in the Upper Chagres appear to reflect episodic introduction of large volumes of wood via landsliding, combined with rapid decay of wood and extremely high transport capacity during wet-season floods. Acknowledgements We thank Holdemar Blaisdell, Alcibiades Grajales, and TRAX IEASA for assistance with field work in 2008 and Aquilino Alveo of STRI and Dan Cadol for field assistance in 2009; Russell Harmon for facilitating

this research; and David Dust for assistance with HEC-RAS modeling. Field work by EW and JG was supported by NSF grant EAR-0808255 to EW. Support for the stream gaging was provided by the U.S. Army Research Office through grant W911NF-07-1-0389 to FO. We gratefully acknowledge the Autoridad del Canal de Panama (ACP) for providing access to their hydrologic data. References Abbe, T.B., Montgomery, D.R., 1996. Large woody debris jams, channel hydraulics, and habitat formation in large rivers. Regulated Rivers: Research and Management 12, 201–221. Abbe, T.B., Montgomery, D.R., 2003. Patterns and processes of wood debris accumulation in the Queets River basin, Washington. Geomorphology 51, 81–107. Abbe, T.B., Montgomery, D.R., Featherston, K., McClure, E., 1993. A process-based classification of woody debris in a fluvial network; preliminary analysis of the Queets River, Washington. EOS Transactions of the American Geophysical Union 74, 296. Barnes, H.H., 1967. Roughness characteristics of natural channels. U.S. Geological Survey Water-Supply Paper 1849. Government Printing Office, Washington, D.C. 213 pp. Benda, L., Miller, D., Sias, J., Martin, D., Bilby, R., Veldhuisen, C., Dunne, T., 2003. Wood recruitment processes and wood budgeting. In: Gregory, S.V., Boyer, K.L., Gurnell, A.M. (Eds.), The Ecology and Management of Wood in World Rivers. American Fisheries Society, Bethesda, MD, pp. 49–73. Bocchiola, D., Rulli, M.C., Rosso, R., 2008. A flume experiment on the formation of wood jams in rivers. Water Resources Research 44, W02408. doi:10.1029/2006WR005846. Braudrick, C.A., Grant, G.E., 2000. When do logs move in rivers? Water Resources Research 36, 571–583. Braudrick, C.A., Grant, G.E., Ishikawa, Y., Ikeda, H., 1997. Dynamics of wood transport in streams: a flume experiment. Earth Surface Processes and Landforms 22, 669–683. Bultman, J.D., Southwell, C.R., 1976. Natural resistance of tropical American woods to terrestrial wood-destroying organisms. Biotropica 8, 71–95. Cadol, D., Wohl, E., Goode, J.R., Yaeger, K.L., 2009. Wood distribution in forested headwater streams of La Selva, Costa Rica. Earth Surface Processes and Landforms Neotropical 34, 1198–1215. Calvo, L.E., Ogden, F.L., Hendrickx, J.M.H., 2005. Infiltration in the Upper Rio Chagres basin, Panama. In: Harmon, R.S. (Ed.), The Rio Chagres, Panama: a Multidisciplinary Profile of a Tropical Watershed. Springer, pp. 139–147. Clark, D.B., Palmer, M.W., Clark, D.A., 1999. Edaphic factors and the landscape-scale distributions of tropical rain forest trees. Ecology 80, 2662–2675. Clark, D.B., Clark, D.A., Brown, S., Oberbauer, S.F., Veldkamp, E., 2002. Stocks and flows of coarse woody debris across a tropical rain forest nutrient and topography gradient. Forest Ecology and Management 164, 237–248. Comiti, F., Andreoli, A., Lenzi, M.A., Mao, L., 2006. Spatial density and characteristics of woody debris in five mountain rivers of the Dolomites (Italian Alps). Geomorphology 78, 44–63. Delaney, M., Brown, S., Lugo, A.E., Torres-Lezama, A., Quintero, N.B., 1998. The quantity and turnover of dead wood in permanent forest plots in six life zones of Venezuela. Biotropica 30, 2–11. Freeman, M.C., Pringle, C.M., Jackson, C.R., 2007. Hydrologic connectivity and the contribution of stream headwaters to ecological integrity at regional scales. Journal of the American Water Resources Association 43, 5–14. Gilmour, D.A., Bonell, M., 1979. Runoff processes in tropical rainforests with special reference to a study in north-east Australia. In: Pitty, A.F. (Ed.), Geographical Approaches to Fluvial Processes. Geo Abstracts, Norwich, England, pp. 73–92. Gregory, K.J., Gurnell, A.M., Hill, C.T., 1985. The permanence of debris dams related to river channel processes. Hydrological Sciences Journal 30, 371–381. Gupta, A., 1975. Stream characteristics in eastern Jamaica, an environment of seasonal flow and large floods. American Journal of Science 275, 825–847. Gurnell, A.M., 2003. Wood storage and mobility. In: Gregory, S.V., Boyer, K.L., Gurnell, A.M. (Eds.), The Ecology and Management of Wood in World Rivers. American Fisheries Society, Bethesda, MD, pp. 75–91. Gurnell, A.M., Piégay, H., Swanson, F.J., Gregory, S.V., 2002. Large wood and fluvial processes. Freshwater Biology 47, 601–619. Harrison, J.B.J., Hendrickx, J.M.H., Vega, D., Calvo, L.E., 2005. Soils of the Upper Rio Chagres basin, Panama: soil character and variability in two first order drainages. In: Harmon, R.S. (Ed.), The Rio Chagres, Panama: a Multidisciplinary Profile of a Tropical Watershed. Springer, pp. 97–112. Haschenburger, J.K., Rice, S.P., 2004. Changes in woody debris and bed material texture in a gravel-bed channel. Geomorphology 60, 241–267. Hassan, M.A., Hogan, D.L., Bird, S.A., May, C.L., Gomi, T., Campbell, D., 2005. Spatial and temporal dynamics of wood in headwater streams of the Pacific Northwest. Journal of the American Water Resources Association 41, 899–919. HEC (Hydrologic Engineering Center), 1995. HEC-RAS, River Analysis System, Version 1.1. U.S. Army Corps of Engineers, Davis, CA. Hendrickx, J.M.H., Vega, D., Harrison, J.B.J., Calvo, L.E., Rojas, P., Miller, T.W., 2005. Hydrology of hillslope soils in the Upper Rio Chagres watershed, Panama. In: Harmon, R.S. (Ed.), The Rio Chagres, Panama: a Multidisciplinary Profile of a Tropical Watershed. Springer, pp. 113–138. Houseal, B., 1999. Plan de manejo y desarrollo del parquet nacional Soberania. In: Ashton, M.S., O'Hara, J.L., Hauff, R. (Eds.), Protecting Watershed Areas — Case of the Panama Canal. Food Products Press, New York, pp. 1–11. Ibáñez, R., Condit, R., Angehr, G., Aguilar, S., Garcia, T., Martinez, R., Sanjur, A., Stallard, R., Wright, S.J., Rand, A.S., Heckadon, S., 2002. An ecosystem report on the Panama

E. Wohl et al. / Geomorphology 111 (2009) 149–159 Canal: monitoring the status of the forest communities and the watershed. Environmental Monitoring and Assessment 80, 65–95. Janzen, D.H. (Ed.), 1983. Costa Rican Natural History. The University of Chicago Press, Chicago. Knox, R.G., Ogden, F.L., Dinku, T., 2005. Using TRMM to explore rainfall variability in the Upper Rio Chagres catchment, Panama. In: Harmon, R.S. (Ed.), The Rio Chagres, Panama: a Multidisciplinary Profile of a Tropical Watershed. Springer, pp. 211–226. Komar, P.D., 1987. Selective gravel entrainment and the empirical evaluation of flow competence. Sedimentology 34, 1165–1176. Lam, K.-C., 1978. Soil erosion, suspended sediment and solute production in three Hong Kong catchments. Journal of Tropical Geography 47, 51–62. Larsen, C.L., 1984. Controlling erosion and sedimentation in the Panama Canal watershed. Water International 161–164. Lienkaemper, G.W., Swanson, F.J., 1987. Dynamics of large woody debris in streams in old-growth Douglas-fir forest. Canadian Journal of Forest Research 17, 150–156. Manners, R.B., Doyle, M.W., Small, M.J., 2007. Structure and hydraulics of natural woody debris jams. Water Resources Research 43, W06432 17 pp. Mao, L., Andreoli, A., Comiti, F., Lenzi, M.A., 2008. Geomorphic effects of large wood jams on a sub-Antarctic mountain stream. River Research and Applications 24, 249–266. Marcus, W.A., Marston, R.A., Colvard, C.R., Gray, R.D., 2002. Mapping the spatial and temporal distribution of woody debris in streams of the Greater Yellowstone Ecosystem, USA. Geomorphology 44, 323–335. May, C.L., Gresswell, R.E., 2003. Large wood recruitment and redistribution in headwater streams in the southern Oregon Coast Range, USA. Canadian Journal of Forest Research 33, 1352–1362. Meleason, M.A., Davies-Colley, R., Wright-Stow, A., Horrox, J., Costley, K., 2005. Characteristics and geomorphic effect of wood in New Zealand's native forest streams. International Review of Hydrobiology 90, 466–485. Milliman, J.D., Syvitski, J.P.M., 1992. Geomorphic/tectonic control of sediment discharge to the ocean: the importance of small mountainous rivers. Journal of Geology 100, 525–544. Montgomery, D.R., Abbe, T.R., 2006. Influence of logjam-formed hard points on the formation of valley-bottom landforms in an old-growth forest valley, Queets River, Washington, USA. Quaternary Research 65, 147–155. Nakamura, J., Swanson, F.J., 2003. Dynamics of wood in rivers in the context of ecological disturbance. In: Gregory, S.V., Boyer, K.L., Gurnell, A.M. (Eds.), The Ecology and Management of Wood in World Rivers. American Fisheries Society, Bethesda, MD, pp. 279–297. Niedzialek, J.M., Ogden, F.L., 2005. Runoff production in the Upper Rio Chagres watershed, Panama. In: Harmon, R.S. (Ed.), The Rio Chagres, Panama: a Multidisciplinary Profile of a Tropical Watershed. Springer, pp. 149–168.

159

O'Connor, J.E., Webb, R.H., 1988. Hydraulic modeling for paleoflood analysis. In: Baker, V.R., Kochel, R.C., Patton, P.C. (Eds.), Flood Geomorphology. John Wiley and Sons, New York, pp. 393–402. Pérez, R., Aguilar, S., Somoza, A., Condit, R., Tejada, I., Camargo, C., Lao, S., 2005. Tree species composition and beta diversity in the Upper Rio Chagres basin, Panama. In: Harmon, R.S. (Ed.), The Rio Chagres, Panama: a Multidisciplinary Profile of a Tropical Watershed. Springer, pp. 227–235. Robinson, F.H., 1985. A report on the Panama Canal Rain Forest. Meteorology and Hydrology Branch, Engineering Division, Panama Canal Commission, Balboa Heights, Panama. 70 pp. Scatena, F.N., 1995. Relative scales of time and effectiveness of watershed processes in a tropical montane rain forest of Puerto Rico. In: Costa, J.E., Miller, A.J., Potter, K.W., Wilcock, P.R. (Eds.), Natural and Anthropogenic Influences in Fluvial Geomorphology. American Geophysical Union Press, Washington, D.C., pp. 103–111. Swanson, F.J., Gregory, S.V., Sedell, J.R., Campbell, A.G., 1982. Land–water interactions: the riparian zone. In: Edmonds, R.L. (Ed.), Analysis of Coniferous Forest Ecosystems in the Western United States. Hutchinson Ross Publishing Co., Stroudsburg, PA, pp. 267–291. Swanson, F.J., Bryant, M.D., Lienkaemper, G.W., and Sedell, J.R.,1984. Organic debris in small streams, Prince of Wales Island, southeast Alaska. USDA Forest Service, Pacific Northwest Forest and Range Experiment Station, General Technical Report PNW-166. Swanston, D.N., Swanson, F.J., 1987. Timber harvesting, mass erosion, and steepland forest geomorphology in the Pacific Northwest. In: Coates, D.R. (Ed.), Geomorphology and Engineering. Dowden, Hutchinson, and Ross, Stroudsburg, PA, pp. 199–221. Wohl, E., 2000. Mountain Rivers. American Geophysical Union Press, Washington, DC. 320 pp. Wohl, E., 2005. Downstream hydraulic geometry along a tropical mountain river. In: Harmon, R.S. (Ed.), The Rio Chagres, Panama: a Multidisciplinary Profile of a Tropical Watershed. Springer, pp. 169–188. Wohl, E., Goode, J.R., 2008. Wood dynamics in headwater streams of the Colorado Rocky Mountains. Water Resources Research 44, W09429. Wohl, E., Yaeger, K.L., 2009. A conceptual model for the longitudinal distribution of wood in mountain streams. Earth Surface Processes and Landforms 34, 329–344. Wolman, M.G., 1954. A method of sampling coarse river-bed material. Transactions, American Geophysical Union 35, 951–956. Wörner, G., Harmon, R.S., Hartmann, G., Simon, K., 2005. Igneous geology and geochemistry of the Upper Rio Chagres basin. In: Harmon, R.S. (Ed.), The Rio Chagres, Panama: a Multidisciplinary Profile of a Tropical Watershed. Springer, pp. 65–82. Wyzga, B., Zawiejska, J., 2005. Wood storage in a wide mountain river: case study of the Czarny Dunajec, Polish Carpathians. Earth Surface Processes and Landforms 30, 1475–1494.