| Lerderderg | |
| Name Other: | The Lerdy[1] |
| Nickname: | The Lerdy[2] |
| Map Size: | 250 |
| Pushpin Map: | Australia Victoria |
| Pushpin Map Size: | 250 |
| Pushpin Map Caption: | Location of the Lerderderg River mouth in Victoria |
| Subdivision Type1: | Country |
| Subdivision Name1: | Australia |
| Subdivision Type2: | State |
| Subdivision Name2: | Victoria |
| Subdivision Type3: | Region |
| Subdivision Name3: | Victorian Midlands (IBRA), Western District |
| Subdivision Type4: | Local government area |
| Subdivision Name4: | Moorabool |
| Subdivision Type5: | Town |
| Subdivision Name5: | Bacchus Marsh |
| Length: | 40km (30miles) |
| Source1: | Blackwood Ranges, Great Dividing Range |
| Source1 Location: | Lerderderg State Forest |
| Source1 Coordinates: | -37.4936°N 144.36°W |
| Source1 Elevation: | 474m (1,555feet) |
| Mouth: | confluence with the Werribee River |
| Mouth Location: | within the Melton Reservoir |
| Mouth Coordinates: | -37.7075°N 144.5189°W |
| Mouth Elevation: | 91m (299feet) |
| River System: | Port Phillip catchment |
| Tributaries Left: | The Old River, Goodman Creek |
| Custom Label: | National park |
| Custom Data: | Lerderderg State Park |
| Extra: | [3] [4] |
The Lerderderg River (known locally as The Lerdy[1]) is a perennial river of the Port Phillip catchment, located in the Western District region of the Australian state of Victoria.
The Lerderderg River rises below the Blackwood Ranges, part of the Great Dividing Range, in the Lerderderg State Forest, near the locality of . The river flows generally south by east in a highly meandering course, through the Lerderderg Gorge within the Wombat State Forest where it is joined by two minor tributaries, The Old River and Goodman Creek, before reaching its confluence with the Werribee River within the Melton Reservoir, southeast of Bacchus Marsh. The river descends approximately over its course.[4]
A diversion in the river's course, called the Lerderderg River diversion tunnel, located approximately upriver from Bacchus Marsh, was constructed between 1855 and 1870. The tunnel was driven through a mountain spur in a horseshoe bend, diverting the river, and allowing the exposed river bed to be sluiced for alluvial gold.[5] [6]
The river is known for the Lerderderg Gorge Picnic Ground, camping, bushwalking, and fishing. In earlier days, gold was mined in the upper reaches, and some of the old mines (considered dangerous to enter) can be seen as you travel along the sluice on the east bank, which previously fed into those mines. The water slowed to a trickle during the severe drought of 2007. Even when not flowing, there are still a few popular swimming spots along the river, including "Harry's Hole", "Third Hole", and "The Pit", all of which are in the Lerderderg Gorge.
The river is traversed by the Western Freeway northeast of Bacchus Marsh.[4]