How Can a Lake Have an Inlet but no Outlet?

Let me introduce you to Lake Ewlyamartup (pronounced you-lee-a-mart-up). Regular Katanning Landcare followers will know the place well – a 100 ha hidden gem of Katanning, being slowly but surely rehabilitated by a band of loving locals and supportive funders. But it’s a funny old lake, and its quirks have, in part, lead to its downfall. Water flows into the northern end of Lake Ewlyamartup via the Ewlyamartup Creek, which brings water from 50,000ha of land including the Katanning and Broomehill townsites.

Water flowing into Lake Ewlyamartup from 50,000ha of catchment.

Most lakes then have an overflow creek, usually at the opposite end, for water to continue its journey downstream. Ewlyamartup doesn’t have an outlet.

Lake Ewlyamartup. Inflow water flows from the top left of the map to the inlet at the north (top) of the lake. Note the overflow bypass creek leading off to the north-east to join the Coblinine River, which flows once the lake is full.

When Lake Ewlyamartup is full, extra water simply bypasses completely. It makes a sharp left turn just north of the lake boundary and heads off to the Coblinine River. There is a very subtle patch of slightly higher ground in this overflow channel that won’t allow water past until the lake level is full enough to breach it. Therefore, Lake Ewlyamartup doesn’t flood either – the relief valve for flood water is before the lake itself.

Water making its way to the Coblinine River (top centre), bypassing a full Lake Ewlyamartup.

So, once water is in the lake, how does it get out? Evaporation. The only way out is up. There is high evaporation in a Katanning summer, so water levels drop significantly seasonally – some summers it dries completely. But although water can evaporate, other things like salt, sediment and nutrients can’t. Each year, they are washed into the lake from the Ewlyamartup Creek, and each year they are left behind as the water evaporates. Where a lake with an outlet will flush through regularly, Ewlyamartup can’t flush. So the salt, salinity and nutrients build up, year after year. Before European settlement, the catchment was fully vegetated and run-off into the lake wasn’t an issue. But now post-clearing and agriculture, these undesirables come every year. By 2010, Lake Ewlyamartup was triple the salinity of the ocean, yellow with eutrophication and lined with a thick black sludge of fine sediments in which you could sink up to your knees.

Tractor demonstrating the depth of built-up sludge at Lake Ewlyamartup, at the start of the 2011 Great Sludge Clean-Out.

The solution to giving Lake Ewlyamartup a healthy future is twofold. Firstly the amount of salt, nutrients and sediment entering the lake has to be reduced, and the community is pulling together to make significant improvements to this. More than a quarter of a million trees have been planted since the 2010 crisis, plus other works.

Volunteers planting trees to protect Lake Ewlyamartup.

And the lake needs to be flushed – to remove what’s in there, and allow flushing of future build up. Katanning Landcare is currently working with the Department of Regional Developments Living Lakes program to design and build a gated artificial flushing channel, giving us control of water quality for years to come.

Planning the flushing channel.

Together we’re restoring Lake Ewlyamartup – for the 95 species of birds that live there, and the people who love the place. Will you make a donation to help Katanning Landcare keep up the good work?