Riparian Planting

What

A riparian area is the interface between agricultural land and a river or stream. Planting along these areas is referred to as riparian planting. This area acts as a buffer between the land and the waterway and filters pollutants, such as nutrients and sediment, which would otherwise enter the waterway. Healthy riparian vegetation helps to reduce stream bank erosion and maintain stable stream channel geomorphology. Vegetation also provides shade, which works to lower water temperatures, an advantage for healthy fish and plant life.

Riparian buffers reduce the flow and volume of surface runoff, thereby allowing for nutrient removal. Riparian buffers should be a secondary restorative measure after controlling pollutants at their original sources.

3D-diagram of a riverside and markers for three different zones, showing plants decreasing in size as they get nearer the riverside.

Why

Riparian planting plays an important role in reducing runoff, removing nutrients, and increasing habitat diversity. Riparian planting provides in stream values including channel shading, improved aquatic habitat, and wood and leaf supply to waterways. These plantings enhance landscape aesthetics and also have recreational and cultural benefits, such as harvesting of flax and other plants. Riparian strips which are planted with shrubs and small trees can be more effective than grass strips as they provide bank stabilisation and flood control.
Short-term grazing or other harvesting is recommended to maintain functionality. The following are indications of the expected improvements to waterways from riparian planting:

  • Bacteria and water clarity: These are the first indicators to show improvement. E. coli indicator bacteria counts reduce significantly, and water clarity typically increases within 2 to 4 years as livestock stop defecating in streams and trampling stream banks, as riparian groundcover plants become thicker and better at filtering runoff from pastures, and as tree roots grow to hold the stream banks together.
  • Silt reduction: Fine sediment (silt) deposited on the stream bed may be washed out over the first few years, but it may appear to get worse for a few years after that as the stream banks adjust to the new vegetation type.
  • Temperature: Water temperature decreases over 3 to 20 years (in small to medium-sized streams) as riparian plants grow to shade the stream. The amount of algae and aquatic weed growing on the stream bed also reduces over this time as shade increases.
  • Nutrients: Dissolved nutrients (that cause growth of nuisance algae) are more complex. Phosphate, which mostly binds to soil particles, usually decreases as riparian areas become better filters, and can be expected to decrease within 10 years of riparian fencing or planting. Nitrogen, which mostly dissolves and reaches streams in the form of nitrate via leaching and underground flow paths, may not be reduced noticeably by riparian planting, unless the groundwater flows through the roots of riparian plants, particularly wetland species. Remove gorse from riparian areas as it fixes nitrogen from the atmosphere and will increase nitrogen loss.

References

Collins, K. E., Doscher, C., Rennie, H. G., & Ross, J. G. (2013). The effectiveness of riparian ‘restoration’ on water quality—a case study of lowland streams in Canterbury, New Zealand. Restoration Ecology, 21(1), 40-48. Connolly, N. M., Pearson, R. G., Loong, D., Maughan, M., & Brodie, J. (2015). Water quality variation along streams with similar agricultural development but contrasting riparian vegetation. Agriculture, Ecosystems & Environment, 213, 11-20. Daigneault, A. J., Eppink, F. V., & Lee, W. G. (2017). A national riparian restoration programme in New Zealand: is it value for money? Journal of Environmental Management, 187, 166-177. Hughes, A. O. (2016). Riparian management and stream bank erosion in New Zealand. New Zealand Journal of Marine and Freshwater Research, 50(2), 277-290. Osborne, L. L., & Kovacic, D. A. (1993). Riparian vegetated buffer strips in water‐quality restoration and stream management. Freshwater Biology, 29(2), 243-258. Parkyn, S. M., Davies‐Colley, R. J., Halliday, N. J., Costley, K. J., & Croker, G. F. (2003). Planted riparian buffer zones in New Zealand: do they live up to expectations? Restoration Ecology, 11(4), 436-447. Schmitt, T., Dosskey, M. G., & Hoagland, K. D. (1999). Filter strip performance and processes for different vegetation, widths, and contaminants. Journal of Environmental Quality. 28:1479-1489. Tabacchi, E., Lambs, L., Guilloy, H., Planty‐Tabacchi, A. M., Muller, E., & Decamps, H. (2000). Impacts of riparian vegetation on hydrological processes. Hydrological Processes, 14(16‐17), 2959-2976.