FRANKLIN & MARSHALL COLLEGE Sediment and Nutrient Loads from Stream Corridor Erosion along Breached Millponds Dorothy Merritts, Robert Walter, and Michael A. Rahnis 5/1/2010 Contents 1 Executive Summary ............................................................................................................................... 5 1.1 President Obama’s Executive Order regarding Chesapeake Bay watershed water quality ......... 6 1.2 Stream corridor and stream bank sediment sources from post-dam breach incised streams .... 6 1.3 Stream bank erosion mechanisms ................................................................................................ 7 1.4 Stream bank erosion rates and the concept of Sediment Production Unit (SPU) ........................ 9 1.5 Modeling dam breaches and changing rates of sediment production ....................................... 11 1.6 Estimates of original and remaining volumes of sediment in millpond reservoirs .................... 12 1.7 Total nitrogen and phosphorus in stream banks ........................................................................ 12 1.8 Prognosis ..................................................................................................................................... 13 2 Introduction and Background ............................................................................................................. 14 2.1 Objectives of this investigation ................................................................................................... 14 2.2 Background to dams and reservoir sedimentation and erosion ................................................ 14 2.2.1 Milldams, sediment trapping, and trap efficiency .............................................................. 14 2.2.2 Case Study: Trap efficiency of the 19th c. Eaton-Dikeman paper mill reservoir, Cumberland County, PA ........................................................................................................................................... 16 2.2.3 Conceptual model of post dam breach stream evolution: Incision and knickpoint migration 18 2.3 Gravitational and hydraulic forces on stream banks .................................................................. 20 2.4 Change in reservoir processes and rates of stream bank erosion after dam breaching ............ 21 3 Characteristics and Mechanisms of Stream Bank Erosion .................................................................. 23 3.1 Geomorphic processes of bank erosion ...................................................................................... 23 3.2 Erodibility and critical shear strength of stream bank sediment ................................................ 24 3.3 Freeze-thaw, cohesive banks of silt and clay, and stream bank erosion rates ........................... 24 3.4 Freeze-thaw cycling and the stream bank debris apron ............................................................. 27 4 Site Descriptions and Research Methods ........................................................................................... 28 4.1 Site locations ............................................................................................................................... 28 4.2 Research methods: Stream bank erosion rates .......................................................................... 30 4.3 Research methods: Measuring the concept of channel-normalized sediment production and its spatial and temporal variability .............................................................................................................. 31 4.3.1 The concept of a channel-normalized Sediment Production Unit (SPU) ............................ 31 4.3.2 Converting volume of sediment eroded to mass (tons) ..................................................... 32 ii 4.3.3 Spatial and temporal variability in stream corridor erosion rates ...................................... 32 4.4 Quantifying nutrients stored in stream bank sediments ............................................................ 33 4.4.1 Field sampling and analytical methods ............................................................................... 33 5 Data and Results ................................................................................................................................. 34 5.1 Stream bank erosion rates and dam breaching: Overview ......................................................... 34 5.2 Case studies for measurements of stream bank erosion rates .................................................. 36 5.2.1 Hammer Creek—breached in 2001 .................................................................................... 36 5.2.2 Mountain Creek—breached in 1985 ................................................................................... 37 5.2.3 Conoy Creek—breached ca. 1972±10 yr ............................................................................. 38 5.2.4 Valley Creek—breached ca. 1920 ....................................................................................... 40 5.2.5 Big Spring Run ..................................................................................................................... 42 5.2.6 Little Conestoga Creek, West Branch, Denlingers Mill—breached ca. 1930±10 yr ............ 43 5.2.7 Big Beaver Creek—breached in 1972.................................................................................. 44 5.2.8 Penns Creek—breached in 1968 ......................................................................................... 45 5.2.9 Gunpowder Falls, Hoffman Mill Pond—breached in 1932 ................................................. 46 5.2.10 White Clay Creek—breached prior to 1938 ........................................................................ 46 5.3 Total nitrogen and phosphorus in stream banks ........................................................................ 47 6 Discussion of Results ........................................................................................................................... 47 6.1 Rates of bank erosion and sediment production curves ............................................................ 47 6.2 Modeling dam breaches and changing rates of sediment production ....................................... 50 6.3 Implications for sediment and nutrient loads to streams from stream bank erosion of legacy sediment ................................................................................................................................................. 52 6.4 Estimates of original and remaining volumes of sediment in millpond reservoirs .................... 54 7 Conclusions ......................................................................................................................................... 56 8 Works Cited ......................................................................................................................................... 59 A. County Maps of Historical Dams ....................................................................................................... 130 B. Methodologies .................................................................................................................................. 137 Estimating Change from Digital Orthophotos ....................................................................................... 137 Overview ........................................................................................................................................... 137 Orthoimagery .................................................................................................................................... 137 Aerial Photography ........................................................................................................................... 137 Feature Digitization ........................................................................................................................... 138 iii Feature Attribution ........................................................................................................................... 138 Estimating Volume Change ............................................................................................................... 138 Stratigraphic Horizons and Local Datum Surfaces ................................................................................ 139 Overview ........................................................................................................................................... 139 Interpreting Legacy Sediment Features on Bare Earth Lidar ............................................................ 139 Delineation of Terrace Surfaces with Lidar in ArcGIS ....................................................................... 139 Delineation of Water Surfaces in ArcGIS .......................................................................................... 140 Calculation of Channel Volume ......................................................................................................... 140 Determining Bank Height Distribution .................................................................................................. 140 C. Production by Station ....................................................................................................................... 142 D. Production by Field Area ................................................................................................................... 145 iv 1 Executive Summary This