Putting landscape rehydration skills into practice at Minyumai

Ben spent the week of 10-14 February 2025 at Minyumai Indigenous Protected Area in northern New South Wales, working with the Rangers to implement some of the landscape rehydration skills learned during the training provided by the Mulloon Institute in November. Readers may recall that during training we constructed three types of small scale flow control structures using locally available materials; pin weirs, brush packs and a log-sill bed control structure. The most noticeable change at Minyumai since we were there in November however was how much the country had dried. As the photo comparison below shows, the log-sill structure, which was flowing when first constructed in November, was dry in February, with just a small shallow pool persisting upstream. This was unexpected as the intervening months of December and January are typically quite wet in this region.

The second most noticeable change since November was the massive increase in the abundance of the Pacific golden orb weaver (Trichonephila plumipes), also known as the tiger spider, in the bush at Minyumai. These spiders have a leg span of around 10 cm, weave enormous webs between trees and sit in their webs at approximately human head level from the ground. This made getting around at Minyumai, either on foot or in the all-terrain vehicle, quite… ‘interesting’.

Our main objective for the week was to construct a series of pin weirs along a flow path that we are intending to send more water through in the future. We described this flow path in a previous blog. It runs along the edge of the 1971 clearance footprint and has old fence posts along its length, suggesting it was the edge of a paddock when Minyumai was a working farm. It is unclear if the flow path was deliberately excavated or just developed due to trafficking of vehicles and/or cattle along the fenceline, but it’s linear course indicates that it is not natural. It is clearly channelising flows that would otherwise find a more meandering, shallow and slower path through the recovering swamp forest, and it is eroding and incising.

Each pin weir was constructed by hammering a row of stakes into the ground about 800 mm apart and weaving saplings through them to create a woven “fence” perpendicular to the direction of flow. The pin weirs are constructed from all-natural materials sourced locally at the site, mostly Acacia sp., which were collected either on site or elsewhere on the property where they are very abundant. Lateral branches were trimmed. Branches were woven together until the pin weirs were about 0.5 m high and 4-5 m in length. The intention is that these “flow-through” structures will slow the flow of water, capture debris upstream, aggrade the bed of the channel (i.e. reverse the process of erosion) and disperse shallow water more broadly through the swamp forest. This will help reduce erosion up and downstream, provide more water for small seasonal ponds within the swamp forest and help advantage native wetland plants over the introduced pasture grasses that currently dominate the understory within the former paddock.

Once the flow path above had been protected with pin weirs, we moved to another part of the property. Headwater creeks bring flows from the surrounding ranges into the flatter country of the former paddocks. However, the excavation of drains through the paddocks in the early 1970s accelerated flows in these natural creeks, causing them to erode. This is a challenging problem at Minyumai but we made some progress in addressing it by installing a series of pin weirs in this area. These weirs were narrower at 1-2 m wide and fit snugly into the incised channel, rather than being open-ended. In this steeper, higher energy environment we placed an apron of stones on the stream bed immediately downstream of each weir for scour protection. The intention is that these structures will immediately reverse erosional processes and lead to aggradation (building up) of the stream bed. It will take many years, and the installation of additional structures, to undo the damage in this part of Minyumai.

It was a productive few days at Minyumai with 14 pin weirs installed. The humidity, spiders, march flies and leeches made it tough going at times but the Rangers welcoming and enthusiastic approach makes every trip to Minyumai great fun and very rewarding.

This project is supported by the NSW Environmental Trust and the Bandjalang Clan, who are the Traditional Owners and custodians of Minyumai IPA.