University of Washington Department of Civil and Environn1ental Engineering QUALITY INDICES FOR URBANIZATION EFFECTS IN PUGET SOUND LOWLAND STREAMS C.W. May, E.B. Welch, R.R. Horner, J.R. Kar and B.W. Mar Final Report for Washington Department of Ecology June 1997 Seattle, Washington 98195 Department of Civil Engineering University of Washington Seattle, Washington 98195 QUALITY INDICES FOR URBANIZATION EFFECTS IN PUGET SOUND LOWLAND STREAMS C.W. May, E.B. Welch, R.R. Horner, J.R. Kar and B.W. Mar Final Report for Washington Department of Ecology Water Resources Series Technical Report No. 154 June 1997 Abstract The Puget Sound lowland (PSL) ecoregion contains an abundance of complex and historically productive salmonid habitat in the form of small streams as well as their riparian forests and wetlands. These watersheds are under intense pressure due primarily to the cumulative effects of urban development. Instream habitat characteristics, riparian conditions, physio-chemical water-quality, and biological attributes of 22 PSL streams (120 survey reaches) were studied over a gradient of development levels to determine relationships between urbanization and stream quality and suggest target conditions for management/protection. Urbanization of PSL watersheds has resulted in an increase in the fraction of total impervious area (% TIA) and a decrease in forested area, including a significant loss of natural riparian forests and wetlands. The cumulative effects of a modified hydrologic (disturbance) regime, the loss ofinstream structural complexity, and the alteration of channel morphological characteristics accompanying urbanization have resulted in substantial degradation of instream habitat during the initial phases of the development process. As the level of basin development increased above 5% total impervious area (% TIA), results indicated a precipitous initial decline in biological integrity as well as the physical habitat conditions (quantity and quality) necessary to support natural biological diversity and complexity. The frequency, volume, and quality of large woody debris (LWD) decreased significantly as basin development and riparian encroachment increased. Loss of L WD due to washout and removal, as well as a reduction in L WD recruitment due to loss of mature riparian forest areas, were significant factors. As a result of the reduction in the quantity and quality ofLWD, along with the effects of a modified hydrologic regime, coho rearing habitat was significantly reduced. Salmonid spawning habitat was also degraded by the cumulative effect~ of urbanization. Fine sediment in spawning gravels generally increased as urbanization increased, while intragravel dissolved oxygen (IGDO) also decreased during the period of salmonid embryo development. Chemical constituents (primarily metals) of water quality during baseflow conditions, as well as storm events, were insufficient to have produced adverse i effects in streams with low to moderate % TIA, but increased markedly in highly urbanized basins (TIA>45%). Results suggest that resource management should place a high priority on preservation and protection of high quality stream ecosystems (TIA <5%) that currently support natural salmonid populations (coho and cutthroat). Mature, riparian forests dominated by coniferous trees should be the long-term management goal. A wide (>30 m) and near-continuous (<2 breaks/Ian) riparian zone appears to be a necessary, although not a wholly sufficient condition for a natural level of stream quality and biotic integrity. Restoring the natural hydrologic regime should be a primary goal for rehabilitation and enhancement efforts. A set of stream quality indices and instream habitat target conditions are proposed for monitoring and managing PSL streams. ii TABLE OF CONTENTS Page Abstract ................................................................................................................................ i Table of Contents ............................................................................................................... iii List of Figures .................................................................................................................... vi List of Tables······························••,•·············································································.'. ..... xi Acknowledgnients ............................................................................................................ xiii INTRODUCTION ............................................................................................................... 1 METHODS .......................................................................................................................... 4 Study Streams .......................................................................................................... 4 Puget Sound Lowland Ecoregion............................................................................. 8 Stream Classification ............................................................................................... 8 Watershed Characterization ................................................................................... 11 Reference Streams .................................................................................................. 12 Measures or Urbanization ...................................................................................... 13 Imperviousness ........................................................................................... 13 Road Density .............................................................................................. 18 Stream Segnient Delineation.................................................................................. 19 Watershed Characterization ................................................................................... 20 Drainage Density ................................................................................................... 20 Riparian Integrity Assessment ............................................................................... 21 Hydrologic Analysis .............................................................................................. 22 Hydrologic Modeling Methods ···································'.······························24 Physio-chemical Sampling and Analysis ............................................................... 28 Instream Habitat Assessment ................................................................................. 30 Riparian Canopy Conditions .................................................................... .32 Channel Morphology ................................................................................. 32 Streambed Scour and Aggradation ............................................................ 33 Intragravel Dissolved Oxygen (IGD0) ..................................................... .37 Salmonid Rearing Habitat ........................................................................ .39 Qualitative Habitat Assessment ................................................................ .41 Biological Indicators ..............................................................................................42 Coho:Cutthroat Ratio ................................................................................ 42 Macroinvertebrate Field Methods ............................................................ .42 Macroinvertebrate Data Analysis and Metric Testing ............................. .43 Statistical Analysis ................................................................................................. 44 RESULTS ····················································································································47 Watershed. Urbanization ......................................................................................... 4 7 Impervious Area ......................................................................................... 47 Road Density .............................................................................................. 52 Forested vs. Developed Area ..................................................................... 55 iii Basin Hydrologic Regime ...................................................................................... 55 Watershed Drainage-Density ..................................................................... 55 Stormjlow:Baseflow Ratio ......................................................................... 56 Flood Frequency ........................................................................................ 61 Relative Stream Power........................................................................................... 64 Hydro/ogic Observations and Model Predictions ..................................... 64 Relative Stream Power Related to Habitat ................................................ 65 Relative Stream Power Related to Biological Indices ............................... 70 Summary ofEffects ofRelative Stream Power .......................................... 73 Artificial Drainage Network ...................................................................... 73 . Stream Classification ............................................................................................. 75 Channel Morphological Characteristics ................................................................. 7 6 Banifull Dimensions .................................................................................