Streamside Management Zones (SMZs)

6/2/2009 11:25:25 PM

A streamside management zone (SMZ) serves as a natural filter of vegetation adjacent to a natural or manmade water body. These zones, also called riparian zones, reduce erosion by both slowing the flow of surface water runoff and increasing water filtration. These water bodies may include streams, rivers, bayous and lakes. To protect water quality, extra precautions may be necessary in carrying out some forest practices.

Key objectives of SMZs are to protect and maintain the quality of water on forestlands by the following:

  • Maintaining a vegetative filtration strip on ephemeral areas.
  • Providing an adequate canopy of forest cover along all perennial streams to maintain normal water and shade conditions.
  • Minimizing forest soil erosion by maintaining the appropriate amount of residual ground cover or forest cover under various soil and slope conditions.

Timber harvest within the SMZ can be used to increase ground vegetation to benefit the filtering effect of the SMZ. Care should be taken not to compromise the objectives of the SMZ.

SMZs should be provided on perennial and intermittent streams and other water bodies. This includes springheads, oxbows, upland flats and drains bordered by steep or erodible slopes. Any existing drainage structures that over time have come to resemble natural drains are also included.

A perennial stream is one that has a well-defined channel and flows year-round except during periods of extreme drought, when it retains pools of water. Intermittent streams have seasonal flow and a continuous well-defined channel. Ephemeral streams flow during and for a few hours or days after periods of heavy rain, and the stream channel is less recognizable than either perennial or intermittent streams.

Streams designated as scenic rivers will be managed in accordance with state law. See the scenic river section of this site.

SMZ width is dependent on watershed characteristics and the risk of erosion in the SMZ and adjacent area. The risk is increased by sandy soil, steep grade, large watershed size or increasing stream width. Estimated normal flow width is the distance in feet between the water’s edge on one side and the water’s edge on the other. This width will be estimated at a time when the stream is at its normal (low) flow. Normal flow width will be an average for the stream, taking into consideration the stream will widen as it flows farther from its source.

Note: SMZ widths are measured from the top of each bank and established on each side of the stream. Determination of SMZ width should be site-specific and should be made by foresters or other qualified professionals. Soil type, slope gradient, vegetation cover, volume flow and stream classification should be taken into consideration when designing each SMZ.

BMPs for Streamside Management

Along perennial streams, timber can be harvested carefully within an SMZ provided that the filtering effects of the SMZ are not compromised.

  • Take precautions to protect the remaining timber stands within the SMZ.
  • Do not remove trees from banks, beds or steep slopes if removal will destabilize soil and degrade water.
  • Permanent residual tree cover is not required along intermittent and ephemeral streams if vegetation and organic debris are left to protect the forest floor during regeneration.
  • Flag or mark SMZs adjacent to all perennial and intermittent streams and lakes before harvesting.
  • Plan harvests to minimize stream crossings.
  • Locate stream crossings where stream impacts are likely to be minimal.
  • Locate roads, skid trails, fire lanes and logging sets outside the SMZ.
  • To minimize damage, limit harvesting on SMZs and sensitive forested wetlands during abnormally wet periods.
  • Consider using wide-tire skidders, forwarders, cable skidders and tracked equipment to minimize soil disturbance in an SMZ.
  • Construct stream crossings to minimize stream bank and channel disturbance.
  • Cross streams at right angles when practical.
  • Consider using portable bridges for temporary stream crossings.
  • Promptly remove all temporary crossings and restore the site after harvesting is completed.

AVOID

  • Skidding across perennial or large intermittent streams, except over an adequately designed crossing.
  • Excessive skidding within an SMZ.

Suggested SMZ Widths:

  • Intermittent = 35 feet wide on each side
  • Perennial (less than 20 feet wide) = 50 feet wide on each side
  • Perennial (greater than 20 feet wide) = 100 feet wide on each side
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