Journal of African Earth Sciences 55 (2009) 205–213 Contents lists available at ScienceDirect Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci Recent glacial recession and its impact on alpine riverflow in the Rwenzori Mountains of Uganda Richard G. Taylor a,*, Lucinda Mileham a, Callist Tindimugaya b, Leo Mwebembezi b a Department of Geography, University College London, Gower Street, London WC1E 6BT, UK b Water Resources Management, Ministry of Water and Environment, P.O. Box 19, Entebbe, Uganda article info abstract Article history: The limited number and duration of hydrological measurements in the East African Highlands inhibit cur- Received 6 January 2009 rent understanding of the impact of glacial recession on alpine riverflow. From historical records and sur- Received in revised form 29 March 2009 veys conducted in the dry season of 2005 and wet season of 2007, we report (1) recent changes in the Accepted 9 April 2009 terminal positions of large valley glaciers (Speke, Elena) and (2) spot measurements of alpine riverflow Available online 5 May 2009 along altitudinal transects of the principal river (River Mubuku) draining alpine icefields in order to assess the relative contribution of icefields and underlying ecotones to river discharge. Observed acceler- Keywords: ation in the rates of termini retreat of the Speke and Elena glaciers since the late 1960s is attributed, in Glacier part, to the convex–concave slope profile in which these valley glaciers reside. We show that current gla- Tropics Riverflow cial recession has a negligible impact on alpine riverflow. Spot measurements of meltwater discharges Climate change indicate that icefields contribute considerably less than 2% of the river discharge at the base of the Africa Rwenzori Mountains during both dry and wet seasons. An anomalously high specific discharge of the River Mubuku (1730 mm aÀ1) arises from high rates of precipitation exceeding 2000 mm aÀ1 below alpine icefields within Heath-moss and Montane forest ecotones that occupy more than half of the river’s gauged catchment area. For other tropical alpine icefields representing a tiny fraction (<1%) of alpine river catchment areas (e.g. Irian Jaya, Kilimanjaro, Mount Kenya), glacial meltwater discharges are similarly expected to contribute a negligible proportion of alpine riverflow. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction et al., 2006) has led to concerns over the impact of deglaciation on river discharge during the dry season (Gasse, 2002; Thompson Tropical alpine glaciers form important reservoirs of fresh water et al., 2002) and the risk of flooding during the rainy season (Ugan- that store seasonal inputs of precipitation, associated with the da Meteorology Department, 2006). Desanker (2002) asserts that movement of the Inter-tropical Convergence Zone (ITCZ), and sus- several rivers on Kilimanjaro are drying out during the dry season tain meltwater discharges during drier periods. In the South Amer- ‘‘due to the loss of the frozen reservoir”. In contrast, Temple (1968) ican Andes, glaciers serve to regulate alpine riverflow upon which and Kaser et al. (2004) contend that glacial meltwater discharges downstream communities rely for year-round water supplies provide insignificant contributions to alpine riverflow in the (Bradley et al., 2006). Meltwater discharges from alpine icefields Rwenzori Mountains and Kilimanjaro, respectively. covering 10% of the mesoscale Rio Santo basin of Peru are esti- The limited number and duration of hydrological measure- mated to provide at least 12% of the annual river discharge (Mark ments in the East African Highlands inhibit current understanding and Seltzer, 2003) and 40% of the dry-season river discharge of the impact of shrinking icefields on alpine riverflow. In the (Mark et al., 2005). Glacial recession since the 19th century has Rwenzori Mountains, no measurements of riverflow have previ- led to increased seasonality and overall reductions in riverflow ously been collected above its base with the exception of a few (Mark and Seltzer, 2003; Bradley et al., 2006). spot measurements of glacial meltwater discharges summarised In the East African Highlands, the observed reduction in areas by Temple (1968). We report spot measurements of riverflow in covered by glaciers since the early 20th century (e.g. Hastenrath, the Rwenzori Mountains along altitudinal transects of the principal 1984; Kaser and Noggler, 1996; Kaser and Osmaston, 2002; Taylor river (River Mubuku) draining alpine icefields during both dry and wet seasons. We use this new dataset together with historical re- cords to assess the relative contribution of icefields and underlying * Corresponding author. Tel.: +44 207 679 0591; fax: +44 207 679 0565. ecotones to river discharge. We also report on recent changes in E-mail address: [email protected] (R.G. Taylor). the terminal positions of regularly monitored valley glaciers. 1464-343X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2009.04.008 206 R.G. Taylor et al. / Journal of African Earth Sciences 55 (2009) 205–213 2. Hydrology of the Rwenzori Mountains The precise onset of modern glacial recession in the Rwenzori Mountains remains unclear. Several authors (Hastenrath, 2001; The Rwenzori Mountains are situated within the western arm of Mölg et al., 2003, 2006; Kaser et al., 2004) contend that deglacia- the East African Rift System (Fig. 1a) and comprise an uplifted tion started around 1880 AD as a result of an abrupt reduction in block (horst) of Precambrian crystalline rocks. Occupying an area precipitation inferred from a reconstruction of the levels of Lake of 3000 km2, the horst is bounded to the north and south by gra- Victoria over the 19th century (Nicholson and Yin, 2001). Recent bens that contain Lakes Albert and Edward, respectively. The palaeolimnological evidence (Russell et al., 2008) suggests that gla- Rwenzori horst tilted up in an ESE to WNW direction (Fig. 2), cial recession started slightly earlier between 1860 and 1870 AD 4 km above the surrounding peneplain in the Late Pliocene (Taylor similar to other tropical alpine icefields. Field research carried and Howard, 1998). Alternating cycles of glacial and fluvial erosion, out in the 1950s (Menzies, 1951; Bergstrøm, 1955; Whittow extensively reviewed by Osmaston (1989), produced deeply in- et al., 1963), early 1990s (Kaser and Noggler, 1991, 1996; Talks, cised valleys. Catchment areas draining eastwards are considerably 1993) and from 2003 to 2005 (Taylor et al., 2006) indicate that larger than those draining to the west due to the orientation of the the area covered by alpine glaciers has reduced from 7.5 km2 in horst’s tilt. From moraine evidence, Osmaston (2006) estimates 1906 to <1 km2 in 2003. that glaciers covered 260 km2 between 10 and 20 ka before pres- Glaciers currently occur on three mountains: Stanley, Speke and ent (BP) (Last Glacial Maximum) and 10 km2 between 100 and Baker. Over the last century, glaciers and their meltwater flows 200 a BP (Little Ice Age). formed headwaters of primarily three rivers in the Rwenzori 0o 25'N,29o 51'E 0 1km Republic of Uganda L. du Speke o 0 N C B 3962m inset (b) 4267m A Masindi 4572m D Fort Portal 1 H Mbarara G Bigo F Bogs Kabale (a) (c) E 2 inset (c) (b) X I J K L. L. Bujuku Vert L Q M O P 4572m 4267m N 3962m Kitandara Lakes X’ 0o 21'N, 29o 54'E glaciers legend A: Speke; B: Vittorio Emanuele; C: Grant; D: Johnston glacial extent (1955) E: East Stanley; F: West Stanley; G: Margherita; H: Alexandra; I: Moebius; J: West Elena; K: Elena; glacial extent (1990) L: Coronation; M; Savoia; N: Edward; O: West Baker; P: East Baker; Q: Moore lake / wetland drainage stream discharge measurement site River Mubuku Basin glacial meltwater flows River Semiliki Basin Fig. 1. (a) Map of Uganda showing the location of the Rwenzori Mountains in Uganda and regional meteorological stations; (b) LandSat7 satellite image showing the Rwenzori horst in relation to Lakes George and Edward as well as the orientation of the cross-section (X–X0)inFig. 2; and (c) map of glacial extent and drainage in the Central Rwenzori Massif (redrawn and adapted from Osmaston and Kaser, 2001) showing the direction of meltwater discharges and location of river discharge monitoring stations. R.G. Taylor et al. / Journal of African Earth Sciences 55 (2009) 205–213 207 WNW ESE -1) 5 1000 1500 2000 2500 3000 5 4 Afroalpine zone 4 Heath-moss forest 3 3 Montane forest 2 Grasslands 2 1 River Semliki Lake George 1 ? 0 0 elevation (km above mean sea level) above (km elevation 10 20 30 40 50 60 70 80 90 100 transect distance (km) Fig. 2. A cross-sectional sketch of the Rwenzori horst tilting upwards from the ESE to WNW (adapted from Osmaston (1989)); vertical exaggeration is  4. Alpine precipitation observed from 1951 to 1954 by Osmaston (2006) and the main vegetation zones (ecotones) are indicated. Z 1 Mountains: the River Mubuku that flows eastward in the Republic Q ½ct À cb@t ¼ M ð1Þ of Uganda and Rivers Butawu and Lusilube which drain westward 0 in the Democratic Republic of Congo (Figs. 1c and 3). Of the NaCl was used as a tracer since applied concentrations and remaining glaciers in the Central Rwenzori Massif (Fig. 1c), most durations of exposure would not be deleterious to the freshwater of the largest including East Stanley (E), Speke (A), Vittorio Emanu- biota. Concentrations of aqueous NaCl can also be easily and reli- ele (B), and Margherita (G) glaciers form headwaters of the River ably measured using an electrical conductivity (EC) meter. All EC Mubuku. measurements compensated for substantial changes in water tem- Precipitation in the Rwenzori Mountains is bimodal; wetter perature (3–22 °C) within the alpine environment and were re- periods occur from March to May and August to November.