43 CHAPTER FOUR STAGE ONE : BASELINE SURVEY OF RIVER CHARACTER AND BEHAVIOUR IN BEGA CATCHMENT 4.1 Introduction and outline of this chapter Stage One of the River Styles framework has three steps as noted in Figure 4.1. Figure 4.1 Steps in Stage One of the River Styles framework STAGE ONE, STEP ONE: Assess regional and catchment setting controls ⇓ STAGE ONE, STEP TWO: Define and map River Styles ⇓ STAGE ONE, STEP THREE: Interpret controls on the character, behaviour and downstream patterns of River Styles In this case study, the following products are presented in separate sections: Stage one, Step One • Regional setting, including landscape units, longitudinal profiles, geology, land use history, flood history etc., and measurement of catchment morphometric parameters. Stage One, Step Two • Catchment-specific River Styles tree. • Catchment-wide map showing the distribution of River Styles. • River Styles proformas, annotated cross sections and geomorphic unit maps and photographs for each River Style. Stage One, Step Three • Summary table of controls for all River Styles. • Longitudinal profile diagrams with associated plots that outline primary controls on each reach for a representative example of each downstream pattern of River Styles. 44 4.2 Stage One, Step One: Bega catchment regional setting 4.2.1 Catchment morphometry and geology Bega catchment is a relatively small coastal catchment located on the South Coast of New South Wales (NSW) (Figure 4.2; Table 4.1). The catchment comprises two hydrologically and geologically distinct river systems, the Brogo to the north, and the Bega-Bemboka to the south. The River Styles framework was applied to 16 subcatchments namely the Bemboka-Bega and Brogo trunk stream systems, Greendale, Frogs Hollow, South Wolumla, Wolumla, Towridgee, Candelo, Reedy, Tantawangalo, Sandy, Colombo, Pollacks Flat, House, Double and Numbugga tributary systems (Figure 4.3). Table 4.1 Bega catchment regional setting Latitude/Longitude 150o00’ East; 36o30’ South Geology Devonian granites and granodiorites of the Bega Batholith with pockets of Ordovician metasedimentary rocks to the north Catchment size • Moran’s Crossing = 312 km2 • Bega trunk stream to Brogo confluence = 1,014 km2 • Brogo trunk stream to Bega confluence = 797 km2 • Bega catchment to tidal limit = 1,840 km2 Channel length Bega trunk to Bega township ~ 60 km Average channel slope • Uplands atop the escarpment = 0.022 m/m • Escarpment = 0.044 m/m • Rounded foothills = 0.005 m/m • Lowland plain = 0.0008 m/m Discharge Moran’s crossing (gauge data) Bega/Brogo River confluence (data 1 in 2 year = 188 m3s-1 extrapolated using the Rational Method) 1 in 5 year = 253 m3s-1 1 in 2 year = 577 m3s-1 1 in 10 year = 764 m3s-1 1 in 5 year = 773 m3s-1 1 in 50 year = 1,234 m3s-1 1 in 10 year = 4,039 m3s-1 1 in 100 year = 1,389 m3s-1 1 in 50 year = 9,997 m3s-1 1 in 100 year = 13,180 m3s-1 Average annual • Escarpment and upland zones = >1,050 mm/yr rainfall • Rounded foothills and along the lowland plain = 750-800 mm/yr • Summer/autumn maxima Vegetation and land • Native, dry sclerophyll, open forest associations in the escarpment and use upland zones. • 70 % of the catchment has been cleared for grazing. Pasture and exotic vegetation dominate the rounded foothills and lowland plain. 45 Figure 4.2 Location of Bega catchment QUEENSLAND Tw e ed Brunswick Richmond 0100200km Clarence Coffs Harbour N Bellinger/ Nambucca Macleay Hastings NEW SOUTH WALES Hunter Manning Karuah Macquarie Lakes Hawke sbury-Nepean S ydn ey B asin/ G eorges Illawarra streams River Shoalhaven Clyde Northern River Moruya Territory Tu r oss Queens- Western land river course Snowy Bega Aus trali a South VICTORIA Aus trali a New cat chment bo unda ry South Tow a mb a Wal es Sydney state border East Gippsland Victoria Tasmania Figure 4.3 Primary river courses Bega catchment Bega valley 36o40’S 150o00’E Scale Creek 05 1015 20 25 kilometres BEGA N Bega Escarpment South Wol uml a 46 Bega catchment has an amphitheatre shape. The landscape is dominated by the escarpment, which surrounds the catchment. The escarpment rises gradually to the north of the catchment where it attains an elevation of 1070 m. Of the 16 subcatchments, only the Bega-Bemboka trunk stream and Candelo and Tantawangalo Creeks have significant areas that drain from atop the escarpment. All other subcatchments drain directly from the escarpment zone itself. Coincident with the rise in elevation of the escarpment, the maximum elevation of the subcatchments generally increases in a northerly direction from Wolumla through to Bemboka and into the upper parts of the Brogo catchment where elevation is generally over 1000 m. Most creeks in the catchment are short, extending less than 30 km from source to the trunk stream confluence (Table 4.2). An exception is Tantawangalo Creek which extends past the escarpment into the uplands. The Bemboka-Bega trunk stream is around 80 km long which is roughly equivalent to the length of the Brogo River. Brogo Dam occurs about a third the way along the stream line, but disconnects around half the catchment area from downstream (around 400 km2). The Bega River and Brogo River trunk streams are both 7th order streams. The Bega River below the Brogo River confluence is an 8th order stream. The tributary systems in the catchment are all 5th or 6th order streams with the exception of Greendale Creek which is a 4th order stream. The amphitheatre shape of Bega catchment results in the convergence of several primary tributaries along a relatively short length of the Bega-Bemboka trunk stream (see Figure 4.4). Sandy Creek, Tantawangalo/Candelo Creek and Wolumla Creek all join Bemboka-Bega trunk stream within around 10 km of the lowland plain. As such, their cumulative impacts (in terms of water and sediment transfer) are manifest along a short reach. For example, flood peaks in Bega tend to occur within 24 hours of rainfall in the upper catchment. Table 4.2 Bega catchment morphometric parameters Topological linear Geometrical linear measurements Other measures measurements Subcatchment Drainage Stream Catchment Stream Maximum Minimum Percent Average pattern order area Length elevation elevation vegetation rainfall (km2) (km) asl (m) asl (m) clearance Bega Dendritic 7 @ 1040 38 1240 Tidal limit 70 875 Bega 8 below Brogo Greendale Dendritic 4 12 5 350 65 20 810 Frogs Hollow Dendritic 5 44 14 380 18 70 800 South Wolumla Dendritic 5 25 10 610 58 70 780 Wolumla Dendritic 6 131 21 776 15 80 813 Towridgee Dendritic 5 14 7 530 110 80 850 Candelo Dendritic 6 114 31 880 75 58 875 Reedy Dendritic 5 22 10 640 115 50 875 Tantawangalo Dendritic 6 326 60 980 45 40 890 47 Sandy Dendritic 6 95 26 860 65 85 800 Colombo Dendritic 5 51 13 1050 175 60 775 Bemboka Dendritic 6 114 44 1240 185 40 1000 Pollacks Flat Dendritic 5 36 11 1070 170 30 860 Brogo total Dendritic 7 797 89 1120 <10 40 1000 House Dendritic 6 35 13 710 40 30 875 Double Dendritic 6 156 34 950 26 40 875 Numbugga Dendritic 5 46 14 810 82 70 875 Figure 4.4 Three dimensional oblique view of Bega catchment – viewed from the Pacific Ocean 4.2.2 Geology and landscape evolution The geology of Bega catchment is dominated by Devonian granites and granodiorites of the Bega Batholith (Figure 4.5). Exceptions include parts of the Brogo catchment. The upper Brogo, House and Double Creeks are underlain by Devonian metasediments. A pocket of Ordovician metasedimentary outcrops in the far north-west of the catchment. Tertiary volcanics, comprising basalts, are found in upper Candelo, Tantawangalo and Bemboka subcatchments. Quaternary alluvium (comprised primarily of sands and occasional gravels) lines most of the primary drainage lines in Bega 48 catchment, especially along the lowland plain. The surrounding foothills of the lowland plain are dominated by Ordovician sediments. Figure 4.5 Geology of Bega catchment 49 The uplift of the eastern highlands began around 95 million years ago (Ollier and Pain, 1994). During the Late Cretaceous, rifting and sea floor spreading, associated with the separation of the Lord Howe Rise from eastern Australia, formed a new continental edge creating a passive continental margin (Ollier and Pain, 1994). Headward fluvial erosion initially created deep gorges and valleys which eventually coalesced via irregular scarp retreat to create the main escarpment (Ollier, 1982). Subsequent denudation lead to the formation of rounded foothills downstream of the escarpment. The morphology of the escarpment within Bega catchment is a result of differential erosion due to mineralogy rather than structural control. The more elevated western margin of the Bemboka Granodiorite is felsic and often ademallitic in composition, while the Bega Valley is often more mafic and easily eroded (Tulau, 1996). This likely enhanced the formation of the amphitheatre catchment shape. The ancestral Bega River drained to the Pacific Ocean via Wallagoot Lake and Penooka Swamp (Nott et al., 1991). Coastal lowlands formed in mid-Oligocene to early Miocene when terrestrial sediments 10-60 m thick were deposited in response to sea level rise (Nott et al. 1991). This deposition caused the diversion of lower Bega River through an incised gorge to the north (named Bottleneck Reach) (Nott et al. 1991). Holocene sediments have been deposited since sea level stabilised at or near the present level about 6000 years ago. This instigated an extension of coastal zones, estuary sedimentation and changes to river structure (eg. formation of backswamps, terraces etc.). More recently, anthropogenic disturbance has instigated considerable landscape change and river metamorphosis (Brooks and Brierley, 1997, 2000; Fryirs and Brierley, 1998 a; Brierley et al., 1999).