APPENDIX L Historic Changes to Wetlands and the Stream Channel Network. Sonoma Ecology Center Arthur Dawson Elisabeth Micheli

Transcription

1 APPENDIX L Historic Changes to Wetlands and the Stream Channel Network Sonoma Ecology Center Arthur Dawson Elisabeth Micheli

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3 Contents Objective... L-1 Methods... L-1 Data Sources and Description...L-1 Results and Discussion... L-2 Historical Data Sources... L-4 Tables Table L-1. Overview Comparing mid-19 th Century Mapped Features and Modern Features for Sonoma Valley Watershed...L-3 i

4 OBJECTIVE We hypothesized that disturbances due to historical land uses introduced by Mexican and American settlers in the mid-19 th century created major changes in the stream and wetland network of Sonoma Valley. This hypothesis was based on preliminary observations of channel incision and studies of hydrologic and geomorphic changes associated with intense grazing, logging, and stream channel dredging conducted elsewhere in northern California. Our primary goal was to superimpose historical stream maps over the contemporary drainage network in a GIS environment to detect major changes in the mapped presence or absence of stream channel features. METHODS Maps from the Mexican and early American eras cover 70% of the watershed. These maps were scanned at a resolution of 300 dpi (or greater) and georectified using the most recent USGS topographic quads as a basemap. Geographic locations which could be identified and located on both historic and modern maps were chosen as reference points. Using the Arcview/Geographic Information System (GIS) mapping program, these reference points were tied from the historic maps to modern coordinates, thus georectifying the historic maps. While accuracy varied widely depending on early mapping and surveying techniques, georectification enabled us to identify the creeks shown, and to locate these and other historic features to within 50 to 500 yards of their actual locations (depending on the particular map). Features on the georectified maps were then traced in GIS, resulting in the creation of an integrated map of the historic watershed. Where possible, other historic sources were consulted to verify the existence of features. Finally, historic and modern features were classified, and changes assessed and quantified to create the final map and the overview table (Table L-1). Data Sources and Description Several types of primary historical sources were consulted, compared, and integrated: Maps from the Mexican and Early American eras ( ). Essentially these documents were made to establish land ownership and covered the areas claimed under Mexican land grants as well as the Pueblo of Sonoma. Accuracy varies widely, but generally increases during this period. Sonoma County Survey, This source was used for a section of Sonoma Creek upstream of Glen Ellen, especially because it documents a now-disappeared meander. Other sources confirm the existence of this feature. Jose Altimira s 1823 journal includes descriptions which were useful in verifying and adding detail to the map data. California Land Court testimony for Los Guilucos Rancho included descriptions of the Kenwood Marsh area confirming the lack of a channel for Sonoma Creek suggested by the earliest map of the rancho. L-1

5 Appendix L Historic Changes to Wetlands and the Stream Channel Network Modern sources used for comparison with historical data included: U.S.G.S. topo maps. GIS stream layer created by the Sonoma Ecology Center. Personal, on-the-ground knowledge of the region and its history was of great importance to the work of interpreting the early maps. Field inspections of features of interest, including old bridge sites, present or abandoned channel locations, and present and former wetland areas were made. RESULTS AND DISCUSSION Over the last 200 years, both direct alterations and land-use patterns have extended the channel network in terms of total length, connectivity, flood conveyance, and access to sediment sources. We hypothesize that the channel network of Sonoma Creek has become entrenched through the valley floor and alluvial fans, leading to an overall loss in wetland, floodplain, secondary channels, and other backwater habitats. The picture of apparent changes drawn from historic sources is consistent with field observations made by geomorphologist Laurel Collins in her current sediment studies (Collins pers comm 2004). Of the tributary channels mapped during the Mexican and Early American periods, approximately 80 percent are shown as directly connected to the main stem of Sonoma Creek (Map 8). The remaining 20 percent displayed an absence of a channel across what likely would have been a wet alluvial fan complex. New channels add 20% length directly to the network. Increased channel connectivity increases the entire channel network by 50%. We show a loss of all secondary channels mapped in the 19 th century. Approximately 2 miles of secondary channels were mapped parallel to the main stem on the historic maps, while today these are entirely abandoned. Two small historic meanders (essentially the only two sinuous parts of the channel) have cut-off. Scarcely any of the mapped wetland features remain, suggesting losses of nearly 1000 acres of freshwater march/wetlands, including 400 acres of perennial lakes, and 90% or 15,000 acres of historical tidal marsh. Field measurements of terraces at bridge sites suggest the main stem of Sonoma Creek has been incising at a rate of approximately 1.0 ft. per decade since before the turn of the 20 th century. Entrenched fan reaches of tributaries display similar extents of incision, particularly on the streams draining Sonoma Mountain on the west side and the valley. Channel entrenchment provides a mechanism which would explain the formation of new channels at the base of alluvial fans which previously may have been unchannelized wetland features. The elimination of back-channel and other slow-water habitats reduces available winter and summer rearing habitat. In particular, the loss of off-channel wetlands and secondary channel L-2

6 Appendix L Historic Changes to Wetlands and the Stream Channel Network habitats mean that there may be only very small quantifies of slow velocity protected habitats for rearing. Further work could establish multiple lines of evidence to corroborate, or disprove, apparent changes shown. Table L-1. Overview Comparing mid-19 th Century Mapped Features and Modern Features for Sonoma Valley Watershed STREAM CHANNEL NETWORK Percent Change (%) Miles Mapped, Historic Miles, Corresponding Modern Total channels Primary (main stem) channel Small meanders Secondary channels Main stem distributary Tributaries WETLANDS Connected to main stem Disconnected from main stem New channels Percent Change (%) Acres Mapped, Historic Acres, Modern Freshwater wetlands -94* Freshwater marsh Ponds Seasonal lake Perennial lakes Tidal marsh ,111 1,586 *The value of 58 acres includes Kenwood Marsh and Ledson Marsh. Percent change does not include ponds or seasonal lakes, for which modern values are undetermined. These figures do not include freshwater marsh which has been converted to ponds or reservoirs. L-3

7 Appendix L Historic Changes to Wetlands and the Stream Channel Network HISTORICAL DATA SOURCES Altimira, Jose Diario de la expedicion verificada con objecto de reconocer terrnos para la nueva planta de la Mision de Nuestro Padre San Francisco principiada le dia 25 de Junio de (Translation by Robert S. Smilie in The Sonoma Mission; The Founding, Ruin and Restoration of California s 21 st Mission. Valley Publishers. Fresno, CA.) Anonymous. c Tereno Nombrado Guilucos Solicitado por Juan Wilson. Bancroft Library, University of California, Berkeley. Map. Anonymous. c Agua Caliente. Bancroft Library, University of California, Berkeley. Map. Anonymous. c Rancho de Huichica Sonoma. Bancroft Library, University of California, Berkeley. Map. Boggs, William California Land Court Transcript, Los Guilucos Rancho. July 31. Bancroft Library, University of California, Berkeley. Describes conditions in the 1840s. Collins, Laurel Personal communication to Arthur Dawson, SEC of preliminary findings of Sonoma Sediment Study. O Farrell, Jasper Map of the Land of Petaluma. Sonoma County Recorders Office, Santa Rosa, California. Peabody, E.J Guilucos Rancho, Surveyed & Drawn for Hood & Petit. Bancroft Library, University of California, Berkeley. Map. Reynolds and Proctor Illustrated Atlas of Sonoma County, California. Reynolds and Proctor, Santa Rosa, CA. Sonoma County Public Works Bridge Reports. Sonoma County Library History Annex. Sonoma County Public Works As-built diagrams for many of the current bridges over Sonoma Creek. Sonoma Index-Tribune January 27 and May 26. Sonoma Index-Tribune Drowning of T.J. Sullivan Narrow Escape of Fred Justi. The County Bridge Carried Away by the Raging Waters of Sonoma Creek. February 1. Sonoma Index-Tribune The Storm: Sonoma Creek Booming County Road at Foot of Broadway in Danger. March 25. Sonoma Index-Tribune Untitled item which begins: The recent storm... January 28. Sonoma Index-Tribune, The Storm s Havoc. January 27. Sonoma Index-Tribune A Prolonged Storm. January 23. L-4

8 Appendix L Historic Changes to Wetlands and the Stream Channel Network Thompson, Thos. H Historical Atlas Map of Sonoma County California. Thomas H. Thompson & Co. Oakland. vongeldern, Otto Plan of Sonoma. Bancroft Library, University of California, Berkeley. Map. Watkins, Carleton E Photographic Views of El Verano and Vicinity, Sonoma Valley, California L-5