Sediment in drains and streams is largely made up of soil (clay, silt and sand) and gravel. A sediment trap is an area where the runoff from a paddock will collect and settle for sufficient time to allow any sediment in suspension to drop out before the water drains away through an overflow or spill way. Heavy, coarse material will drop quickly, while the finer material that holds most of the nutrients will take longer. Any measure that spreads water out and slows down the flow, will allow sediments to drop out.

There are many different types of sediment traps.

• Ponds are generally the most common type and are formed by excavation into the ground or by the construction of an embankment. They are designed so water leaves at a rate that will allow suspended sediment to settle out.

• Drains can and do catch sediment. Sections of a drain can be engineered to increase the amount of sediment that they collect.

• Swales are relatively flat grassed areas with gently sloping sides and a gentle longitudinal slope. They are generally used to transport runoff following a heavy rain. Water flows at a low velocity so that the grass acts as a filter to remove sediment.