FOUR KEY FACTORS

There are four key factors that should be considered in the creation or management of drawdown zones. These are water level control, substrate conditions, shoreline topography, and the practicalities of vegetation establishment.

WATER LEVEL CONTROL

The timing and magnitude of drawdown and flooding events can be manipulated to favour the establishment of certain plants and inhibit the growth of others. Different durations of drawdown will favour different species, as will the season when it occurs. Due to its effects on invertebrates, plants and substrate, drawdown can be used as an effective tool to manage fish and attract birdlife.

SUBSTRATE CONDITIONS

Substrate character is one of the main factors determining the establishment and growth of plants on lake and reservoir shorelines. Wave action reduces the organic matter content of the substrate and prevents plant growth by uprooting seedlings and damaging adult plants. Management should seek to reduce wave action, through design and provision of shelter, and also reduce the effects of waves by the provision of reefs, booms or geotextiles on the shore.

SHORELINE TOPOGRAPHY

The development of shoreline topography needs to be carefully planned and implemented to create the right conditions for the desired wetland communities. Habitat design needs to make sure that the drawdown zone is extended to produce a large area of variable hydrology and high biological diversity, by the creation of hummocks and hollows or ridge-and-furrow, the development of shallow slopes and the bunding of bays or inlets.

VEGETATION ESTABLISHMENT

The use of appropriate species and management techniques, tailored to the situation and used at the correct time of year, can create vegetation communities that will be able to survive, and benefit from, the environmental factors of flooding and exposure. Planting, seeding and the introduction of rhizome ‘mulch’ have all been used in drawdown zones, but aftercare is often key to the success of these schemes. If water level control is possible, the development of plants from the wetland seedbank can often be possible and allow a sustainable vegetation to develop.