River Region Analysis

ATTACHMENT H STREAM CLASSIFICATION REPORT, TITTABAWASSEE RIVER Stream Classification Report, Tittabawassee River, Midland, MI 1. INTRODUCTION TO STREAM CLASSIFICATION For more than a century, the Tittibawassee River has undergone numerous changes in its floodplain and channel as a result of anthropogenic influence. Floodplain impacts have included removal of vegetation, changes in vegetation density and type, alterations in landforms and construction of encroachments that block the flow-path of flood flows, including roadway embankments and pipeline crossings. The channel itself has been impacted by changes in its boundary conditions, including removal of vegetation and artificial hardening with levees and sheet-pile walls. The construction of several dams upstream of the Midland Plant has likely created conditions of “clear water discharge”, changing the rivers sediment supply downstream. All of these anthropogenic influences have likely, in some way, affected river stability and sediment supply, thereby creating the potential for negative impacts in the form of excess erosion and deposition. The extent, magnitude, frequency and duration of the responses of the Tittibawassee River to these influences is a question which must be answered through a systematic, quantitative assessment of historic land use changes, channel processes, sediment supply and future evolution of the river. Essential to this assessment is the ability to separate natural and anthropogenic influences, and to predict how the river will reach an equilibrium state through a balance of energy and sediment supply (Rosgen, 2006). While this assessment necessarily involves numerous levels of data collection, analysis and prediction, it should begin with a broad-level classification of the river. Such a classification helps provide an overview of the general state of the river, allows separation of the river into areas of distinct morphology, and elucidates future data collection and analysis needs. For this investigation, the Rosgen Level I and Level II classification methodology provided the framework for this classification. However deviations have been necessary due the lack of field- verified bankfull elevations. Since Level I and Level II classification does not require a great deal of field survey data, the approximations made for this classification are a reasonable deviation from standard methodology. It is not expected that acquisition of more detailed field verified data will alter the results of the classification for the River Regions discussed below. Later, more detailed levels of classification will incorporate a greater amount of data collection and involve a higher level of analysis that comports with the river assessment standards established by the US Environmental Protection Agency (USEPA) and the Rosgen methodology. 1.1 Description of Methodology Stream classification is a useful tool because it provides a common language for describing and discussing stream and river systems (in the following discussion of stream classification the terms stream and river are used interchangeably). A variety of stream classification systems have been developed over the years. A widely used classification system based upon key morphological features such as channel pattern, slope, and shape has been developed by Rosgen (1996). 2/15/08 1 The Rosgen classification methodology uses four assessment levels that vary from a broad geomorphic characterization down to a very detailed description and assessment. The Level I assessment describes the geomorphic characteristics that result from the integration of basin relief, land form, and valley morphology. The dimension, pattern, and profile of rivers are used to delineate geomorphic types at a coarse scale. Many of the Level I criteria can be determined from topographic and landform maps and aerial photography. The Level I stream classification serves four primary functions: • Provides for the initial integration of basin characteristics, valley types, and landforms with stream system morphology. • Provides a consistent initial framework for organizing river information and communicating the aspects of river morphology. Mapping of physiographic attributes at Level I can quickly determine location and approximate percentage of river types within a watershed and/or valley type. • Assists in the setting of priorities for conducting more detailed assessments and/or companion inventories. • Correlates similar general level inventories such as fisheries habitat, river boating categories, and riparian habitat with companion river inventories. The advantage of a broad, general classification is that it allows for a rapid initial delineation of stream types and illustrates the distribution of these types that would be encountered within a given study area. The Level I classification and delineation process provides a general characterization of valley types, and identifies the corresponding major stream types, A through G. Illustrations of the Level I stream types are shown in Figure 1. Figure 1. Level I Stream Types 2/15/08 2 Whereas Level I stream types are identified on the basis of the valley landforms and channel dimensions observable on aerial photos and maps, Level II stream types are determined with field measurements from specific channel reaches and fluvial features within the river's valley. The Level II classification process employs more finely resolved criteria in order to address questions of sediment supply, stream sensitivity to disturbance, potential for natural recovery, and channel response to changes in flow regime. Level II stream type delineation criteria are based on of channel cross-section, longitudinal profile, and planform features as measured and computed from collected field data. Many of the Level II criteria are based upon the bankfull discharge or bankfull stage of the stream. 1.2 Bankfull Discharge (Channel-Forming Discharge) and Stage River form and fluvial processes evolve and function through mutual adjustments toward self- stabilization (Rosgen, 1996). The physical appearance of a stream or river is a product of the adjustment of the river’s boundaries to the magnitude of stream flow and sediment produced from an attendant watershed. The individual river characteristics are further modified by the influence of channel materials, basin relief, and other features of valley morphology along with a local history of erosion and sediment deposition. The most important stream process in defining channel form is the bankfull discharge. Bankfull discharge is the flow that transports the majority of a stream’s sediment load over time and thereby forms and maintains the channel. The bankfull stage and its associated discharge serves as consistent morphological indices which can be related to the formation, maintenance, and dimensions of the channel as it exists under the modern climatic regime. The term “bankfull” was initially used to describe the incipient elevation on the bank where flooding begins. In many stream systems, the bankfull stage is associated with the flow that just fills the channel to the top of its banks and at a point where water begins to overflow onto a floodplain. However, if a stream has become incised because of changes in the watershed or anthropogenic changes to the channel or floodplain, the bankfull channel is often located at an elevation that is lower than the top of bank. Bankfull stage can be observed and determined within entrenched streams by using a series of common stage indicators, such as a bench or scour line, which may be observed along the boundary of the bankfull channel. An illustration of bankfull in relation to a stream channel and other floodplain features is shown in Figure 2. A commonly accepted definition of bankfull provided by Duane and Leopold (1978) is “that the bankfull stage corresponds to the discharge at which channel maintenance is the most effective, that is, the discharge at which moving sediment, forming or removing bars, forming or changing bends and meanders, and generally doing work that results in the average morphologic characteristics of channels.” It is this discharge along with the range of flows that make up an annual hydrograph which govern the shape and size of the channel. 1.3 Stream Classification Terminology Stream dimensions, patterns, and bed features associated with the river are generally described as a function of channel width measured at bankfull stage. The following provides the definition of many of the terms used in classifying a channel: 2/15/08 3 • Bankfull Width (Wbkf): Width of the channel at bankfull stage elevation in a riffle section. • Bankfull Depth (Dbkf): Mean depth of the channel cross-section at bankfull stage elevation in a riffle section. • Bankfull Cross-Section Area (Abkf): Area of a channel at bankfull stage elevation in a riffle section. • Width/Depth Ratio: Bankfull width divided by bankfull depth in a riffle section. • Width of Flood-Prone Area (Wfpa): Width at elevation of twice maximum bankfull depth. Entrenchment Ratio (ER): A computed index value which is used to describe the degree of vertical containment of a river channel (width of the flood prone area at an elevation twice the maximum bankfull depth/bankfull width). • Sinuosity (k): Defined as stream length/valley length or valley slope/channel slope. • Meander Width Ratio: A secondary delineative criteria defined as meander belt width/bankfull width that describes the degree of lateral channel containment, and is primarily used

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