by FatCanyoner » Fri 22 Jan, 2021 2:56 pm
As Tom correctly points out, much of the water found flowing in Blue Mountains creeks is effectively spring fed. This process is visible wherever you see a hanging swamp (and there are many of them in the upper mountains). Water flows through the sandstone, hits a layer it can't get past, then slowly moves along that plane until it reaches the surface. These reliable seeps then support hanging swamps.
Sandstone country drains very quickly, so most rain flows off within a matter of a couple days. Most of the water that continues to flow after this is water seeping out of the rock or held back by swamps. That's the reason why, even deep in drought, many creeks still continue to have a trickle of water. It's also what keeps those hanging swamps lush and green throughout summer. Once you disturb that underground water movement -- such as with longwall coal mining -- swamps and perennial streams end up running dry. There's several clear examples of this on the Newnes Plateau, which is quite tragic.