Week 4 - Connections to the Land

Required Readings

Bob Bilby, Tim Beechie

January 27, 1999

Freshwater hydrology - Tim Beechie

Terrestrial Habitat Influences - Bob Bilby

1) Morphology and evolution of salmon habitats ­ post glaciation.

Most river valleys in the Puget Sound basin are filled with sedimentary deposits dating from >14,000 years B.P. Deposits of till, outwash, and lacustrine clay indicate a series of advances and retreats that periodically created ice dams at the mouths of valleys (clays deposited in lakes created by ice dams, outwash deposited by pro-glacial rivers, and till under the ice sheet). Since that time, erosion of these deposits created a valley morphology characterized by terraces up to elevations of 2000 feet. Radiocarbon dating indicates that most of the erosion occurred prior to about 8,000 years B.P. in the S. F. Stillaguamish valley.

Valley morphologies we see today are partly a function of the underlying bedrock terrain. Where valleys are narrow, most deposits have been eroded away and terraces are few. In wider valleys, we see broad terraces (several miles wide many valleys). The resultant topography influences the distribution of salmonid habitats. In constrained valleys we find steeper tributaries and narrower flood plains, which means not much low-gradient stream that provides habitat for salmon. Where valleys are wider, we tend to see long reaches of moderate-gradient tributaries cutting through terraces, and extensive low-gradient channels on flood plains.

2) Habitat losses in the last 150 years.

An assessment of habitat losses over the last 150 years in the Skagit and Stillaguamish River basins shows that the greatest overall losses are due to isolation of habitats by levees, channel alterations, and culverts blockages. Tributary degradation is most severe in urban areas, with lesser degradation in agricultural and commercial forest areas. Lower river off channel habitats (side-channel and distributary sloughs) have been most severely impacted because they are located in floodplains that have been claimed for agricultural and urban development.

3) Physical processes that form salmon habitat.
Sediment supply is most significantly affected by changes in mass wasting rates. Stands less than 20 years old have mass wasting rates approximately four times that of mature forest areas, and roads have mass wasting rates approximately 40 times that of mature forest areas. The cumulative effect in a watershed is to increase average annual sediment supply under a forest management regime to about twice that of the natural fire regime. Large increases in bed load supply fill pools, and changes in fines may reduce survival of eggs in the gravel. The rate of sediment transport out of reaches (annual travel distance of bed load) averages about 20 channel widths per year in channels with slope less than 0.03. In one example (Deer Creek, North Fork Stillaguamish) pool depths recovered significantly less than 10 years after they were filled by landslide sediments.

LWD recruitment has been altered by previous logging practices and conversion to agricultural and urban land uses. Compared to projected LWD recruitment under the natural fire regime, forest management typically reduces recruitment of LWD large enough to form pools by an estimated 35 to 100%. Pool spacing is more sensitive to LWD loading in channels with slope between 0.02 and 0.05 than in channels with slope less than 0.02. In low-slope channels, pool spacing is partly maintained by lateral scour at banks when LWD is absent. Recovery rate for recruitment of LWD large enough to form pools is primarily a function of channel size and the tree species colonizing a disturbed riparian area.

Hydrologic regime is most dramatically affected by land use in urban areas. Increased impervious area dramatically increases peak flows. Impervious area <3% has an insignificant effect; impervious area >10% significantly alters stream morphology and biota. Effects of forestry activities (roads and timber harvest) on peak flows are generally restricted to less than bankfull flows. Altered channel morphology or species assemblages are (so far) difficult to document. Other affects on hydrology are water withdrawals and hydroelectric dams.