Restoration of floodplain topography by sand-splay complex formation in response to intentional levee breaches, Lower Cosumnes River, California

Abstract

Restoration of sustainable geomorphic processes that create floodplain topography through development of sand-splay complexes at intentional breaches is one method to promote variability in physical structure needed for habitat restoration. The topography of splay complexes provides a range of floodplain elevations that creates local variability in (i) inundation duration and frequency and depth to ground water that influence riparian vegetation establishment; and (ii) flow depth and velocity that create refuge for fish. Two intentional levee breaches along the lowland Cosumnes River, Central Valley, CA, were evaluated during water years 1999 and 2000 in order to document changes in morphology and relief associated with deposition of sand-splay complexes. During the study period, annual peak-flow recurrence intervals ranged from ∼1 to 3 years, and water flowed through the breaches for a minimum of 55 days during water year 1999 and 53 days during water year 2000. At the two study sites, rapid vertical accretion and scour occurred within the first several years after intentionally breaching the levee at the Accidental Forest floodplain (constructed in 1995) and at the Corps Breach floodplain (constructed in 1997). Splay complexes are organized into a variety of landforms, including lateral levees and lobes separated by new floodplain channels. Maximum deposition measured on the splay surface is 0.36 m/year, while maximum scour in channels is 0.27 m/year. Juxtaposition of floodplain splay deposition and adjacent channel scour creates relief ranging from ∼1.6 to 0.25 m that decreases with distance from the breach and that becomes more pronounced over time as higher magnitude floods scour channels in the old floodplain sediment and deposit new sand and silt onto the surface of the splay. The ratio of splay complex height to depth of formative flow is estimated as ∼0.4. Progradation of main and secondary splay channels takes place by down-floodplain sand transport (25 m/year maximum). Large wood recruited onto the floodplain through the breach promotes local scour and deposition that enhances topographic variability. At one of the study sites, initial grading of a low setback berm prior to opening the breach forced a change in floodplain flow direction and the geometry of the splay complex. Additionally, progradation of the complex is arrested by an excavated pond that creates a sediment trap. We present a conceptual model that describes the importance of floods in constructing and modifying sand-splay complexes that create floodplain topography. The potential habitat variability created as floodplain topography evolves is the linkage between physical and ecological processes that are critical for restoration.

Introduction

Floodplain topography, the distribution and relief of floodplain landforms, constitutes the physical structure underlying floodplain ecosystems. As such, floodplain topography is an important consideration in the restoration of lowland rivers. In this paper, the use of the term “restoration” is not meant to imply full recovery to the predisturbance condition. Rather, it is intended to imply a trajectory toward sustainable physical processes consistent with the term “rehabilitation” (Federal Interagency Stream Restoration Working Group, 1998). Periodic erosion and sedimentation during floods sustain the health and productivity of floodplain river ecosystems Junk et al., 1989, Bayley, 1991, Bayley, 1995, while floodplain habitat plays a critical role in sustaining riparian ecosystem diversity Ward and Stanford, 1995, Stanford et al., 1996. Thus, it follows that the dynamic geomorphic processes that form and modify floodplain topography are a fundamental component of riparian ecosystem ecology. Sand-splay and channel complexes, often called crevasse splays, create variability in floodplain topography by depositing sediment and cutting channels during avulsion at natural levee breaches in anabranching rivers Smith et al., 1989, Richards et al., 1993. Over time, sand-splay complexes evolve toward a topographically diverse floodplain system containing channels, wetlands, and levees Smith et al., 1989, Smith and Perez-Arlucea, 1994. Splay deposition also occurs at accidental breaches in engineered levees along channelized rivers, such as the Mississippi and Missouri Rivers, during the 1993 flood Jacobson and Oberg, 1997, Schalk and Jacobson, 1997 or intentional breaches in engineered levees in Mississippi Delta distributary channels (Boyer et al., 1997).

This paper reports results of a field investigation during 1999 and 2000 documenting sand-splay complex deposition and scour and initial development of floodplain topography resulting from two intentional breaches in levees separating the Lower Cosumnes River, CA (Fig. 1), from its floodplain. Prior to anthropogenic disturbance, the lowland Cosumnes River was an anastomosing river that contained multiple channels, seasonal marshes, and “lagunitas,” or perennial floodplain lakes. The dominant geomorphic process of change in this system was probably avulsion (Florsheim and Mount, 1999). Historical data suggest that most of the floodplain was covered by dense riparian forest patches or belts Thompson, 1977, The Bay Institute of San Francisco (TBI), 1998, while tule marshes dominated lowest elevation areas (Commission of Public Works, 1861). Over the last 150 years, riparian forests were cleared, floodplain lakes, sloughs, and marshes were filled, and topography was graded to support agriculture. At present, much of the lower Cosumnes River is concentrated into a single channel constrained by levees, agriculture dominates the floodplain, and only isolated remnants of the former aquatic-terrestrial ecotone remain. In this paper, a “level floodplain” is defined as a floodplain lacking topographic relief, but which still has a gradient in the downstream direction.

The Cosumnes River is the largest river lacking large dams draining the west slope of the Sierra Nevada, and because flow is not regulated, the lower Cosumnes River floodplain is an ideal location to test methods to restore floodplain habitat. As part of a long-range watershed plan (TNC, 1992), the first intentional breach was completed in the Cosumnes River Preserve at the “Accidental Forest” floodplain in the fall of 1995, and the second was completed in the fall of 1997 at the “Corps Breach” floodplain (Fig. 2). The 1999–2000 study period reported in this paper represents the fourth and fifth year of sand-splay complex evolution at the Accidental Forest and second and third year of evolution at the Corps Breach study areas. The impetus for these restoration activities on the Cosumnes River Preserve arose after an unintentional levee breach in ∼1985 deposited a ∼0.06-km² sand splay on a farmed field adjacent to the river (Fig. 3). An “Accidental Forest” of cottonwood (Populus fremontii) and several species of willow (Salix) grew on the sand splay and demonstrated that in addition to restoring the hydrologic connectivity between the river and the floodplain, intentional levee breaches could be an effective method to promote floodplain revegetation.

The purpose of this field study was to document the geomorphology associated with the intentional levee breaches including (i) initial development of floodplain topography created by the morphology and relief of the sand-splay complexes; and (ii) changes in morphology resulting from processes such as sediment deposition, new floodplain-channel incision, breach scour, and progradation of the splay complex. Our results also analyze the effect of large wood recruited through the breaches that add variability to the splay complexes, sediment textures that compose the splay deposit, and the effect of initial grading on the morphology of the splay complexes. Based on this work, we present a conceptual model that illustrates the development of morphology and relief within the splay and channel complexes.

Sand-splay deposition in the previously level, tilled floodplain areas on the Cosumnes River Preserve is a first step in the development of floodplain topography needed to restore physical diversity to the floodplain restoration areas following reintroduction of water to the floodplain. Pervasive floodplain land uses often level floodplain topography and construct levees that inhibit floodplain inundation, erosion, and sedimentation. Consequently, the results presented in this paper are significant for potential floodplain restoration and management initiatives in other lowland river systems in California's Central Valley and in other floodplain river systems with levees.