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.
Camping on the northern foreshore of the lake is popular.
Toilet facilities are available.
Dogs are permitted at lake.
Swimming, water skiing and jet skis are permitted.
Camping is permitted in designated camping areas only.
Shooting of ducks is permitted during duck season, subject to seasonal restrictions.
Camp fires are permitted in designated fireplaces.
| Current Level | 185.050 m AHD |
| Current Volume | 15,310 ML |
| Percent Full | 29% |
| Current Inflows | -960 ML/week |
| 24 hour Rainfall | 0 mmCummulative Inflows Graph |
| Current Releases | 44 ML/day |
| Current Spills | 0 ML/day |
| Current Airspace | 37,990 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 Volume | 187.12 m AHD |
| Spillway Length | 53,300 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 | 23.4 km2 |
| Major Tributary | Mt William Creek and Fyans Creek |
| Average Annual Inflow | 52,000 ML/year |
| Maximum Depth | 4.5 m |
| Year Constructed | 1903 |