[Interior Watershed Assessment Procedure Guidebook Table of Contents]

Appendix 10.

Forestry impacts on floodplains and riparian zones and directions on filling in the riparian forms

Floodplains and riparian zones

Floodplains are areas of flat land bordering a watercourse. They are frequently at, or near, the same elevation as the top of the streambanks and are subject to flooding. Riparian zones are defined in the Forest Practices Code as the land adjacent to the normal high water line in a stream, river, lake, or pond and extending to the portion of land influenced by the presence of the adjacent ponded or channeled water. Commonly, the riparian zone is associated with fish-bearing streams. Although fish-bearing stream reaches usually have gradients lower than 12%, streams in floodplain riparian zones are much less steep, usually having gradients less than 3%.

Floodplains are formed by both sediment deposition during overbank flooding and by channel erosion (e.g., streambank cutting along the outside of bends) and sedimentation (e.g., sediment deposition along the inside of the bend). Because floodplains are built by historically repeated cycles of erosion and deposition, they commonly have many flood channels (back channels) associated with the main channel. Smaller tributary channels and side channels also flow along the floodplain and through the riparian zone. These smaller channels are critical to certain phases of the life cycle of salmon. For instance, coho salmon fry that remain in the stream for up to two years after emerging from the spawning beds use the back channels as refuge from floods. These channels are often small and not always obvious, particularly to those individuals not aware of their importance or trained in their identification.

Floodplains and riparian zones are also influenced by watershed conditions in headwater areas. Sediment moved downstream to the floodplain zone can create large sediment wedges (large accumulations of stream sediments) in the channel. These can cause the stream course to change direction, in turn leading to floodplain erosion and deposition. The sediment wedges move slowly downstream during large floods, leading to changes along the floodplain for very long time periods.

The riparian zone is of critical importance to stream ecosystems. The riparian vegetation: contributes nutrients and fish food by providing plant material and insects to the stream, regulates stream water temperatures (tree canopy shading), and delivers large woody debris (LWD) to the stream. The LWD provides much of the fish habitat and also contributes to stream channel stability. The roots of streamside vegetation tend to resist stream erosion by helping to hold the bank materials together. Streamside vegetation promotes overbank sediment deposition and also provides hiding cover or refuge for fish.

Forestry effects

The floodplain is susceptible to impacts due to several different forestry activities. Logging roads, harvested areas, and camp facilities can occupy the entire floodplain area of a watershed; impacts can be locally intense and also spatially dispersed. Roads are often located on the floodplain, and these cross, and sometimes block, the numerous small channels. This has been shown to be detrimental to aquatic organisms. Mainline roads often parallel the main stream channels and this can lead to many problems over long lengths of fish-bearing streams. The impacts range from streambank destabilization, to increased surface erosion and sedimentation, and on to dewatered channels, obstructions to fish passage, and altered channel locations. Bridges and other crossings can also lead to erosion and sedimentation.

Other forestry activities affect floodplain conditions. Logging in riparian zones has led to increased bank erosion, loss of in-channel islands, increased size and frequency of sediment wedges, and altered stream shape. Logging camps, storage areas, and dumps are commonly located in floodplain areas because of the relative ease of access and construction and the readily available source of drinking water. These facilities have caused stream pollution problems, as well as changes to the stream channel itself.

Stream channels

Streams are watercourses formed when water flows between continuous definable channel boundaries. Streams range in size from very small in headwater areas to very large in downstream areas of a watershed. The stream flow can be either perennial (continuous all year) or intermittent. Streams commonly flow through floodplain materials previously deposited by the stream itself.

Natural processes

The general characteristics of a stream and its channel conditions, including its shape and appearance, depend on many factors, including: Several secondary factors also determine the appearance of stream channels:

Forestry activities and stream channel conditions

Forestry activities influence some, but not all, factors that control channel conditions. Logging can influence flood characteristics (see Appendix 8), sediment delivery (see Appendices 9 and 11), and the nature and extent of riparian vegetation. Typically, if stream flows and sediment delivery to the channel are increased, it is expected that the channel would become wider, shallower, less sinuous, and steeper (within limits, depending on sinuosity). Changes in sediment supply to the channel can have a major influence on in-stream biological conditions. For instance, increased sediment supply can result in reduced fish rearing and overwintering habitats (loss of pools and underbank areas), decreased juvenile fish survival and smolt production, and impaired spawning and incubation environments (degraded riffle sites). Changes in the species, size, amount, distribution, and orientation of LWD also have a significant effect on stream channel conditions (e.g., pools can infill and riffles can become more extensive).

The influence of logging will also vary depending on stream size. Small streams can be affected directly by landslides, particularly in headwater areas, and this can result in complete disruption of the normal shape of the channel. Medium-sized channels are usually influenced strongly by in-stream woody debris. The LWD characteristics are influenced by both streamside and upslope logging. Altered LWD characteristics have been shown to lead to changed channel morphology, sediment characteristics, and hydrologic conditions. Logging activities have less obvious direct influence on the larger stream channels. Exceptions include direct disturbances of streambanks (crossings, streamside logging, yarding, etc.), bed conditions (obstructions, sediment, and debris removal), and mid-channel islands.

Filling in the riparian impacts form

Portion of stream logged (indicator #9): To obtain this value, measure the total length of streams in the sub-basin that have been logged right to the edge (i.e., no riparian buffer) and divide by the total length of all streams in the watershed. The total length of streams in the sub-basin includes all those streams that appear on a 1:20 000 scale TRIM or forest cover map. A section of stream "logged right to the edge" is identified on a forest cover map by locating cutblocks that are immediately adjacent to or straddle a stream. Thus, the assumption here is that if a cutblock is mapped as being immediately adjacent to the stream or straddling the stream, then no riparian buffers were left.

Portion of fish-bearing stream logged (indicator #10): This value is obtained in the same way as indicator #9, but only fish-bearing reaches are considered. Fish-bearing reaches are defined in Appendix 7. For this indicator, the length of fish-bearing stream reaches logged right to the edge is divided by the total length of fish-bearing stream reaches, not the total length of all streams.

The level 2 analysis does a more in-depth analysis of the condition of the floodplain and the stream channel.

[Return to top of document]