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3.0 Scientific Basis And Underlying Principles 3-7 April 2004 3.4 Types Of Floodplains Floodplains in the Upper Colorado River Basin are classified as depressions, terraces, and gravel pits. Gravel pits are mechanical excavations that often function as depressions (Figure 3- 2; Irving and Burdick 1995). A fundamental understanding of the hydrological and biological chronology of these floodplains is important to coincide management of these floodplains with appearance and development of the larval fish. All three features may become inundated during high spring runoff and may dry and reset in summer. The degree of inundation varies among floodplain sites, depending on the magnitude of runoff and the ground elevation that separates the feature from the main river channel. Depressions and gravel pit ponds are typically separated from the main channel by an elevated levee that is either natural or manmade. An undesirable feature of gravel pits is the often deep excavation below river bed elevation that retains water permanently and becomes a long-term refuge and source of nonnative fishes. Terraces are sloping features that fill and drain with changes in river stage. The current management strategy for depressions and gravel pit ponds is habitat restoration through either partial removal of levees or one or more breaches in a levee to allow flooding at lower river stages (Lentsch et al. 1996a; Utah Division of Wildlife Resources 1996; Flo Engineering 1997; Nelson and Soker 2002). Floodplain Depression Floodplain Terrace Gravel Pit Pond (/) cn~ CCll .t:; ..J Q.~ (/)0 u: (/) I-Q) Cll> ECll E..J ~~ u: (/) cQ) Eai ~..J :i~ <0 u: Figure 3-2. Schematic of the bed profile of the three major floodplain classifications at various flow regimes in the Upper Colorado River Basin (excerpted from Irving and Burdick 1995).