Lake Lonsdale is a large, shallow reservoir located on the Mt William Creek. It has a very large catchment area. The Mt William Creek catchment is not reliable during times of drought but can be a substantial producer of water during wetter years.
Lake Lonsdale was originally developed to supply the Wimmera-Mallee Domestic and Stock channel system, however its role has changed considerably following completion of the Wimmera Mallee Pipeline. Lonsdale continues to play a significant role in the overall water supply system; is a key source of water for the environment; and is highly valued for its recreational use.
The lake is seen as a key recreation facility for the residents around Stawell.
Toilet facilities are available.
Dogs are permitted at lake.
Swimming, water skiing and jet skis are permitted.
Camping permitted in designated areas.
Seasonal duck hunting permitted.
| Current Level | 184.670 m AHD |
| Current Volume | 10,620 ML |
| Percent Full | 20% |
| Current Inflows | 145 ML/week |
| Current Airspace | 42,680 ML |
| Full Supply Level | 187.62 m AHD |
| Full Supply Volume | 65,480 ML |
| Maximum Operating Level (MOL) | 187.12 m AHD |
| Maximum Operating Volume | 53,300 ML |
| Spillway Level | 187.12 m AHD |
| Spillway Length | 244 m |
| Secondary Spillway Length | 440 m |
| Secondary Spillway Capacity | 149,470 ML/day |
| Outlet Capacity | 600 ML/day |
| Catchment Area | 1015 km2 |
| Surface Area When Full | 26 km2 |
| Major Tributary | Mt William Creek and Fyans Creek |
| Average Annual Inflow | 52,000 ML/year |
| Maximum Depth | 4.5 m |
| Year Constructed | 1903 |