Large-Scale Land Management to Prevent and Reduce Alluvial Gully Erosion

Mapped areas of concentrated alluvial gully erosion and soils with high erosion risk (i.e., dispersible or sodic soils along river frontage terraces, Plate 3; Plate 5; Plate 17) should be targeted for large-scale land management changes and localised intensive rehabilitation actions to cumulatively reduce sediment yields to downstream rivers, wetlands, estuaries, coasts, and off-shore reefs. Land management actions to directly or indirectly prevent or reduce alluvial gully erosion from high risk areas include:

  • Increasing perennial grass cover on shallow slopes above gully prone areas such as steep river banks.
  • Reducing water runoff toward gully prone areas by improving soil/vegetation hydrologic functions upslope.
  • Reducing concentrated water runoff down cattle tracks (pads), roads, and fences.
  • Increasing perennial grass cover within gully prone areas (river banks and hollows) to resist erosion and trap sediment.

A paradigm shift and a full suite of altered management actions (cattle, fire, weed, road, fencing) are needed on erosion-prone sodic soils along river frontage, in order to reduce the initiation of new alluvial gullies, slow gully erosion rates where already initiated, and aid in indirect/passive long-term rehabilitation efforts.

Plate 05

Cattle Management along River Frontage with Dispersible or Sodic Soils

Cattle grazing is a primary agent for accelerating gully erosion on highly-erodible sodic soils. Cattle grazing intensity and impacts are often concentrated along river frontage terraces and elevated floodplains. Ideally, permanently maintained cattle exclusion fencing is needed around large concentrated areas of gully erosion to create ‘soil conservation areas’ by fencing according to land type (i.e., sodic soils on river frontage). Several priority river frontage areas have been identified in the Normanby (e.g., the Granite Normanby River, Plate 5; Plate 17). Alternatively, seasonal spelling of cattle could be used if it can be demonstrated to reduce gully erosion. Government or market-based compensation is needed for the economic loss of graziers, such as payment for ecosystem services (carbon, biodiversity, soil retention) or promotion of ‘improved pasture’ management on stable and productive soils.

To document the effectiveness of cattle exclusion for vegetation recovery and soil erosion reduction, cattle exclusion experiments have been initiated around alluvial gullies in the upper Laura-Normanby catchment (Plate 6). Four (4) cattle exclusion areas (3-5 ha) were fenced in 2012 to start a long-term monitoring program (5 to 10+ years). A before-after, control-impact study design was used with vegetation and erosion plots and detailed repeat topographic surveys (LiDAR). Funding is needed for long-term monitoring.

Plate 06

Draft BMPs for Cattle Grazing on River Frontage with Dispersible or Sodic Soils

  • Exclude cattle from mapped sodic soils on river frontage terraces and elevated floodplains.
    • Fence to land type.
    • For best management, complete fence completely around mapped erodible sodic soils, locating fences on more stable geology. 
    • Otherwise, place fences well back from high banks when excluding cattle from immediate river frontage zones, when entire areas of sodic soils cannot be fenced. Include river terraces and flats upslope of gullies (breakaways) inside fence to increase grass cover and reduce water runoff from these catchment areas (Plate 2).  
  • Wet season spelling (Dec-April) of cattle along erodible river frontage, when full cattle exclusion is not feasible.
  • Cattle exclusion or spelling will reduce cattle tracks (pads) over steep banks and across terrace/floodplain flats.
  • Vegetation cover targets >75% at break-of-season (Nov), >1000 kg/ha perennial grass.
  • Install off-stream water points for cattle on stable geology outside of mapped erodible sodic soils and well away from river banks.
  • Reduce cattle numbers during drought. Use Bureau of Meterology (BOM) climate forecasts and market forecasts to destock early for income benefits, soil protection, and prevention of land and pasture degradation.

Fire Management along River Frontage with Dispersible or Sodic Soils

Fire regimes on dispersible or sodic soils in river frontage terraces and elevated floodplains need to be tailored toward maximizing the health of perennial grass cover (for water runoff regulation and soil stability), minimizing weed dominance or spread, and regulating the amount of grass cover consumed by cattle each year to build up grass cover and fuel loads. In some highly eroded locations, fire should be excluded to maximise any potential vegetation cover. At the landscape scale, a return toward a mosaic (patchy) burn pattern and variable timing of fire regimes is needed, locally tailored to soil type and vegetation community (Plate 7). More scientific experimentation is needed to identify the most appropriate fires regimes for sodic soils on terraces and elevated floodplains in river frontage to promote perennial grass health, reduce water runoff and soil erosion, and minimise gully erosion, while also managing the balance between trees and grass.  Some preliminary BMPs are included below. 

Plate 07

Draft BMPs for Fire Management along River Frontage with Dispersible or Sodic Soils 

  • Cattle spelling during the wet season (Dec-April) or full cattle exclusion to improve perennial pasture health and build up fuel loads for appropriate fire regimes, if any.
  • In some highly erodible locations, fire could be excluded altogether to maximise any potential vegetation cover.
  • Reduce the occurrence of intense late-dry season fires through river frontage with sodic soil.
  • Create a mosaic (patchy) burn pattern at the landscape scale with variable timing.
  • Mange fire in a controlled fashion in discrete areas.
  • Locally tailor fire regimes to soil type and vegetation community for specific objectives
    (i.e. increase long-term perennial grass cover, weed control). 
  • Use prescribed aerial and/or ground burning in the early-dry season to install fire breaks and take advantage of natural or made-made fire barriers.
  • Move the location of early-dry season fire breaks every year to burn in a mosaic pattern and not burn the same location repeatedly, especially alongerodible river frontage.
  • Avoid repeatedly using river frontage and riparian zones as fire breaks. Frequent fires can reduce the long-term health of perennial grass and increase gully erosion potential on terrace and floodplain margins.
  • Do not repeatedly grade fence lines as fire breaks, as this will accelerate erosion.
  • Early-wet season ‘storm-burns’ (1-3 days after first >25mm rain) should be used cautiously in strategic patches along highly-erodible river frontage with dispersible or sodic soils to avoid accelerated water runoff and soil erosion at the start of the wet season. Storm-burns should be used for specific localised purposes (rubber vine control; thickening of Melaleuca viridiflora, improving perennial grass germination/health) where long-term improvements of grass cover can be demonstrated to reduce gully erosion.
  • Control cattle grazing of grass re-growth after both early-dry season and early-wet season fires to promote perennial grass health during critical growth periods, ensure good grass cover, and protect soil during early-wet season rainfall.

Weed Management along River Frontage with Dispersible or Sodic Soils

The invasion of exotic weeds into river frontage country and riparian zones has become ubiquitous in the Normanby catchment (Plate 8). Many annual weeds compete with preferred native perennial grasses, provide little ground cover at the beginning of the wet season, have low root density and soil cohesion, and change the infiltration potential of soils. Most weeds have been introduced or increased in river frontage from upstream land use, local disturbance such as over grazing, altered competition dynamics between grass and weeds, intense late-dry season fires, roads and fences. More scientific experimentation is needed on how to control weeds (fire, chemical, mechanical, biological) along river frontage terraces and elevated floodplains with sodic soils to promote perennial grass health, reduce water runoff and minimise gully erosion.

Plate 08

Draft BMPs for Weed Management along River Frontage with Dispersible or Sodic Soils 

  • Spell cattle from river frontage to increase grass competition with weeds.
  • Use grass fuel loads after spelling to periodically use fire at appropriate times/locations for weed control (e.g., rubber vine) and breaking weed seed cycles (e.g., hyptis, grader grass).
  • Use herbicides and other control methods in strategic areas to reduce weed cover.
  • Prevent spreading weeds along roads and fences. Clean vehicles/machines regularly.
  • Prevent spreading weeds with imported hay.
  • Prevent spreading weeds when moving cattle from weeded paddocks to less weeded areas, or between properties. Use holding paddocks with regular weed management to contain weed spread.

Road and Fence Placement on River Frontage with Dispersible or Sodic Soils 

The best long-term solution to road and fence stability is to not locate this infrastructure on dispersible or sodic soils and river frontage areas (Plate 9; Plate 17). Once dispersible or sodic soils are disturbed, erosion is difficult to prevent. Roads and fences should be placed on stable soils and geology, such as on subtle ridge and spur crests between drainage catchments. Maps of soil/geology, topography (including LiDAR), erosion hazards, and aerial photographs (+Google Earth) can be useful for locating infrastructure during property planning and to minimise the number of crossings of existing gullies, unchannelled hollows, creeks, and rivers. This will reduce long-term road maintenance costs and rill and gully erosion. Where roads and fences must be located through erodible river frontage, frequent water diversion structures (whoa boys) should be installed and capped with locally imported soil/gravel, in addition to armouring the approaches to creek crossings to minimise gully erosion. Vegetation clearing should also be minimised, especially on steep banks.

Plate 09

Draft BMPs for Road & Fence Locations on River Frontage with Dispersible or Sodic Soils

  • Avoid building roads and fences through river frontage with dispersible or sodic soils.
  • Locate roads and fences on stable soils and geology, such as on subtle ridge and spur crests between drainage catchments.
  • Minimise the number of road/fence crossings through unchannelled hollows, gullies, creeks, and rivers.
  • Scout and map the best routes for fences using motor bikes, GPS, topographic maps and Google Earth images. Adjust line accordingly to avoid erosion hazards.
  • Install frequent water diversion structures (whoa boys) capped with locally imported angular gravel, in addition to armouring the approaches to channel and hollow crossings.
  • Water diversion frequency depends on slope and soil type, but for highly erodible soils on slopes >5%, spacing should be every 10-25 metres.
  • Minimise vegetation clearing (grass and trees) especially on steep banks.

Vegetation Clearing along River Frontage with Dispersible or Sodic Soils

Tree clearing along river frontage can initiate or accelerate alluvial gully erosion by disturbing soil and vegetation from bulldozing, chaining, and stick raking vegetation, as well as changing the water balance by reducing water transpiration by trees, raising water tables, increasing sub-surface water seepage, and accelerate surface water runoff (Plate 10). Generally, clearing trees from erodible soils on river terraces and floodplains of river frontage should be avoided. Where this has already occurred, mitigation measures can be instated to reduce the continuing or future erosion of alluvial gullies.

Plate 10

Draft BMPs for Vegetation Clearing along River Frontage with Dispersible or Sodic Soils

  • Avoid clearing trees and other vegetation along river frontage with dispersible or sodic soils.
  • Retain or install grass and tree vegetation buffers along with cattle exclusion along drainage lines, hollows, gullies, creeks and rivers, well back from breaks in slope of high banks
  • For small gullies and creeks, buffer widths should be > 50 m wide from the high banks where alluvial gullies often initiate.
  • For larger creeks and rivers, buffers should be > 100 m wide from the high banks to include the local catchment area of gullies and hollows.
  • In practise, uncleared buffers could be > 1 km from the centre of large creek or river channels (Plate 2).
  • Install contour banks on cleared paddocks to manage excess water runoff.
  • Install earthen banks around gully heads to divert water runoff into a safe disposal points.
  • Where tree regrowth after clearing is problematic, use cattle spelling, periodic fire, pasture competition, and chemical weed control to control regrowth, rather than repeated mechanical intervention that can disturb soils and accelerate erosion.