The northern boundary of the Wimmera Region approximates the southern limit of the lighter sandy and sandy loam soils of the Mallee Regions. However, several more or less isolated sandy ridges that support mallee communities occur throughout the region (e.g. in the Wycheproof—Dumosa area). The region is divided into separate eastern and western parts by a north–south aligned sandy ridge that is approximately 0.5 km to the east of Dimboola and approximately 4 km to the east of Gerang Gerung.
The boundary of the eastern part of the Wimmera then follows the Adelaide–Melbourne railway line to Wail (i.e. approximately the eastern edge of the Little Desert sands). Much of the western boundary is shared with the Little Desert (of the Lowan Mallee Region). The first north–south aligned sand-ridge west of Natimuk (crossing the Wimmera Highway approximately 44 km WSW of Horsham) forms the common boundary with the Wannon Region. The southern boundary is shared with the Grampians, Midlands and Wannon Regions. It includes the southern limit of the Wimmera River and Avon River floodplains, excluding the sandy scree and outwash slopes of the Grampians, the bedrock of the Midlands Region, the lateritized tablelands and plains of the Wannon Region to the west of the Black Ranges (south of the Toolondo Reservoir), and the lateritized soils south of Brimpaen and Lah-arum (between the Black Range and the Grampians) that have been included in the Grampians Region. The eastern boundary (shared with the Riverina Region) is arbitrarily defined as the area to the west of the Avoca River alluvial plain.
The western part of the Wimmera Region extends from the southern limits of the Big Desert to the northern limits of the Little Desert. This part of the region extends to and includes the Bordertown environmental association of the Murray Mallee province (sensu Laut et al. 1977a, 1977b).
Major grids C, D, G and H. Approximate area 12 468 km2.
The major landform characteristic of this region is that of the Pliocene to Pleistocene marine sediments of the Murray Basin that have been overlaid by fluviatile deposits (Douglas, this volume). The resulting landscape is a flat to undulating plain raised to only 190 metres above the surrounding plain, with no features above 340 metres in altitude. The predominant soil types in the region are duplex soils which, in the central Wimmera (Dahlen–Murtoa–Rupanyup area) have been weathered to form laterites. From Lake Buloke, through Warracknabeal to near Dimboola, cracking clays (including gilgais, or colloquially ‘crab-hole clays’) are common (Gibbons and Rowan, this volume). Large, almost flat, alluvial plains of the Wimmera and Richardson Rivers, and Dunmunkle and Yarriambiac Creeks (the main riverine systems), dominate the landscape in the southern parts of the region. The Dunmunkle and Yarriambiac Creeks arise as effluents of the Wimmera River. Lake Buloke (near Donald) is the largest in the region. Other lakes include Green, Pine and Taylors Lakes south-east of Horsham.
Climatic variation throughout the region is small, as there are no major topographic variations that would cause marked climatic differences. The summers are typically hot, and the winters are mild. About 60 per cent of the annual precipitation is of low intensity and occurs during the winter months (particularly from May to October). Irregular thunderstorms during summer often produce intense rainfalls of short duration. Annual average rainfall is between about 400 and 500 mm. Severe frosts often occur during winter (see also ‘Climate of Victoria’, this volume).
Nearly all of this region has been alienated and cleared for agriculture (Edwards 1984; Carr in Anon. 1985, map 9). A summary of the vegetation of the region is based on what remains on farms, roadsides and the scattered small flora reserves. Connor (1966b) concluded that the original vegetation of the Wimmera consisted mostly of a savannah woodland with Eucalyptus microcarpa, E. largiflorens, E. leucoxylon, E. camaldulensis (in order of frequency) and Allocasuarina luehmannii as the dominants (Figure 6.13). Insufficient vegetation remains to enable each alliance to be circumscribed. Furthermore, it appears that all savannah woodland communities intermix with each other. Although most woodlands occur as relict populations on small areas of public land and roadsides, sizeable areas of mixed woodland do remain in the Barrett (north-east of Murra Warra), Rupanyup South and Marma (south of Murtoa) Reserves.
Eucalyptus camaldulensis (River Red-gum)
Eucalyptus camaldulensis tall woodlands, or, rarely, open-forests, occur as relatively narrow bands on the floodplains of the major rivers (e.g. near Marma on the Wimmera River, also the Avon and Richardson Rivers), and around permanent swamps (e.g. Dooen Swamp and Lake Buloke). This species often occurs in association with E. largiflorens. The understorey primarily consists of grasses (commonly species of Agrostis, Chloris, Danthonia, Eragrostis and Themeda triandra), and sedges (commonly species of Carex and Cyperus and Eleocharis acuta). Scattered shrubs are also present. Low Eucalyptus camaldulensis woodland occurs infrequently along Yarriambiac Creek.
Eucalyptus largiflorens (Black Box)
Eucalyptus largiflorens forms a woodland to open-woodland community throughout much of the region. It is confined to areas of low-lying clay soils that are subject to infrequent flooding or periodic waterlogging (Carr in Anon. 1985). Since this species is less tolerant of free-standing water than E. camaldulensis the two species replace each other along moisture gradients. Eucalyptus largiflorens frequently forms pure stands (e.g. at Dooen Swamp, near Pimpinio, and along much of the Yarriambiac Creek) or commonly occurs in association with E. camaldulensis (e.g. along the Wimmera River and at Lake Buloke), with E. leucoxylon (e.g. near Marma), with E. microcarpa (e.g. between Drung Drung and the Yarriambiac Creek), or with Allocasuarina luehmannii (e.g. east of Blackheath, and over large areas north of Murtoa). Characteristically this woodland has a grass- or sedge-dominated understorey. The sedges are more common on the wetter sites. Muehlenbeckia florulenta commonly forms a scattered shrubby layer on wetter sites. Occasional trees or small communities of Eucalyptus largiflorens do occur in localized wetter areas of generally better-drained grasslands.
Eucalyptus microcarpa (Grey Box)
Eucalyptus microcarpa commonly forms medium-height (5– 15 m high) woodland communities that are variably associated with E. largiflorens, E. leucoxylon and Allocasuarina luehmannii. This woodland occurs on well-drained, fertile loam soils. Taller woodlands (15-28 m high) occur at Marma and near Rupanyup South, mixed with Eucalyptus leucoxylon, E. largiflorens and E. melliodora. It forms a grassy woodland with Allocasuarina luehmannii south of the Wimmera River, with an understorey typically dominated by exotic annual grasses or by Stipa and Danthonia species. Themeda triandra was probably a common grass that has been eliminated by grazing (Carr in Anon. 1985), although it was probably never common under tree canopies.
Eucalyptus leucoxylon (Yellow Gum)
This species occurs on lighter-textured soils which are widespread but uncommon in the region. Southeast of Dimboola Eucalyptus leucoxylon occurs on the lighter transitional loamy soils, whereas E. largiflorens occurs on the heavier-textured soils at the boundary between the Wimmera and Murray Mallee Regions. E. leucoxylon occurs in association with E. largiflorens, E. melliodora, E. microcarpa and Allocasuarina luehmannii in the Marma forest, and with A. luehmannii and some Eucalyptus largiflorens in the Barrett Timber Reserve (north-east of Murra Warra). Stands of E. leucoxylon also occur between Rupanyup and Glenorchy, particularly near Rupanyup South, and near Mt Jeffcott and Charlton. This species extends south of Glenorchy on solodic soils into the Midlands Region.
Allocasuarina luehmannii (Buloke)
This is the most widespread tree species in the region. Although low Allocasuarina luehmannii woodlands and open-woodlands (5–15 metres high) once covered large areas on the fertile and periodically waterlogged soils of the Wimmera plains, they are now restricted to isolated communities on roadsides, farmland and small areas of public land. Nearly all the remaining stands have been subjected to a long grazing history, and exotic annual grasses now dominate most of the ground-layer (Carr in Anon. 1985). The common grasses of a typical Allocasuarina luehmannii woodland (e.g. near Natimuk) include Avena fatua, A. sativa, Bromus diandrus, B. hordeaceus, Hordeum vulgare, Stipa nodosa and S. rudis. Although scattered shrubs may be present, a true shrub-layer is not developed. Allocasuarina luehmannii frequently is associated with Eucalyptus largiflorens and often with E. leucoxylon (e.g. south of Mt Arapiles, near Marma, and near Murtoa) (Plate 6F), and/or with E. microcarpa (e.g. southeast of Horsham). Scattered pure stands occur in the region (e.g. north of Wail, near Pimpinio, and north of Jung).
Stipa and Danthonia species grasslands are restricted to self-mulching soils in several areas (e.g. between Jung and Murtoa, the Kalkee Plain between Pimpinio and Dooen, and the Lallat Plain east of Rupanyup). These grasslands are now often indistinguishable from the adjacent woodlands which have a grassy ground-layer (Connor 1966b). Grazing and agricultural practices have resulted in the replacement of native grasses by introduced pastures or weeds, such as Avena, Bromus, Dactylis, Critesion, Hordeum, Lolium, Phalaris and Vulpia. Allocasuarina luehmannii occurs as small, often pure, stands in the grasslands (e.g. on the Kalkee Plain).
This region has been extensively cleared for the cultivation of cereal crops, particularly wheat, barley and oats. Most farms also carry some livestock, predominantly sheep. Much of this region receives additional supplies of water via the gravitational-feed channel system that originates in the Grampians (Calder, J. 1987). Small irrigation areas near Horsham and Murtoa support mixed farming, mainly with sheep and dairy cattle.
There are none in the region.