Floodplains: the Biggest Slow-Release Water Source Around

Article and diagrams copyright © Cam Wilson

At the top end of the Marshalls’ property on the Southern Tablelands, NSW, Australia, the creek is bone dry. This spot, fed by 1250 Ha of native forest, has been that way for 10 weeks now.

Meanwhile, 1.2 km downstream at the base of their property, flowing past the fodder poplars, the bamboo and the ferns and dense native revegetation (where only blackberry stood twelve years ago), is one and a half megalitres of the crystal clear water you see in the photo above; every day. Since the creek dried up at the top of their property, 120 megalitres is a conservative estimate of the base flow that ‘the sponge’ that is ‘Sunningdale’ has continued to release to the landscape below. This is despite a catchment increase between the two sites of only 8% and five out of the last six months of rainfall being well below the average.

What’s the catch? If you’d like a bit of background on how a property like Peter and Kate Marshall’s, which has reinstated the original floodplain hydrological processes, is able to store and then slowly release water, check out the simple diagrams below.

Note: This approach is only suited to certain landscape settings. Expert advice should be sought and relevant legislation adhered to when considering works of this kind.

This is a fairly typical erosion gully in the Southern Tablelands. The major erosion happened decades ago, therefore the floor of the gully has mostly revegetated and stabilised. However, the alluvial aquifer remains drained down to approximately the base of the gully.

With the gully incised, in many locations even the largest runoff events remain contained within the channel. For the short period the creek is running high, there is some lateral infiltration into the alluvial aquifer, but it’s often a fairly insignificant amount.

Depending on the local regulations and the order of the stream, porous structures, or leaky weirs of varying heights can be constructed.

Whenever there is sufficient flow from above, the structure causes a pool to form. As well as enabling riparian vegetation to establish, with the associated bed stability and habitat benefits, the raised water level in the pool encourages water to laterally rehydrate the surrounding floodplain.

When flood flows occur, depending on the height of the structure, access to the floodplain is now available once again. With the water spread in a thin sheet across the land, it not only reduces the energy and erosive potential within the channel, but also gives more opportunity for the alluvial aquifer to recharge, with infiltration from above.

In time, depending largely on how porous the floodplain sediment is, the alluvial aquifer will be raised. (The closer to the surface the water table, the more important the flooding process becomes, due to the freshwater lens it creates over the heavier saline groundwater.)

Due to porous nature of structures and floodplain sediments, during extended periods without flow coming into the system, the pools can begin to drop. At such time, water stored in the floodplain begins to feed back into the creek. At the same time, depending on how high the water table has been raised, floodplain vegetation will benefit from moisture available through capillary action.

Back to the 120 Ml released over the past 10 weeks just from the small section the Marshalls have control over. Well, multiply that by the 30km of creek which borders agricultural land in the Jembaicumbene Creek catchment of which they’re a part and you’ve potentially got 3600 Ml of water that didn’t rush off to the ocean back in March and could be utilised during this relatively dry period to kick Spring off in a big way and build natural capital in a variety of ways, limited only by your imagination. Multiply that out by the hundreds of other creeks feeding the Shoalhaven. Multiply that out by the hundreds of rivers across the country and you have a very different set of waterways to what we see today.

Meanwhile, the moisture stored in the floodplain can of course be put to many uses, whether its pasture, agroforestry, productive wetlands or, if you’re like Peter and Kate, a combination of all of the above.

If you’d like to hear more information on the great work of pioneer land managers such as the Marshalls, or if you’d like follow in their footsteps and undertake similar work, visit www.earthintegral.com, or follow us on our facebook page.