Q. Watershed Characteristics ________________________________________________________________ Chapter Watershed Characteristics “Any river is really the summation of the whole valley. To think of Qit as nothing but the water is to ignore the greater part.” --Hal Borland Q.1 Introduction Once a small agricultural community of 14,0001 people, Salinas today is the largest city within California’s Central Coast, and is the Monterey County government seat. Within its approximately 14,400 acres, the City has become a residential, suburban community within an agricultural setting. The City’s population is now a growing 153, 215 people.2 Salinas is also Monterey County’s workforce center, supporting approximately one-third of all jobs within Monterey County. In 2012, agricultural production within Monterey County accounted for nearly $4.14 billion in sales.3 Agricultural production, and the City’s economy and quality of life, rely upon a steady supply of clean water. Waterways in the Salinas Valley are conjunctive, that is, surface water flows recharge groundwater. Groundwater supplies most of the region’s water. Runoff from the Gabilan and Santa Lucia Mountain Ranges are tributary to Salinas River flows. Runoff from local mountains, farms and developed areas can carry with it more than merely water. Runoff can also carry a mix of pollutants. This chapter provides a brief summary of hydrologic conditions within the City of Salinas and nearby watersheds. 1 1950 2 State Department of Finance, May 2013. 3 County of Monterey Agricultural Commissioner’s Office, Monterey County Crop Report, 2012. ______________________________________________________________________ 277 Stormwater Management Plan ______________________________________________________________________ Q.2 Hydrology The River itself has no beginning or end. In its beginning, it is not yet the River; in its end, it is no longer the River. What we call the headwaters is on a section from among the innumerable sources which flow together to compose it. At what point in its course does the Mississippi become what the Mississippi means?” --T.S. Eliot The City of Salinas is located in California’s central coast within Monterey County. It lies within the northern portion of the Salinas Valley flanked by the Gabilan Mountains to the northeast and Santa Lucia Mountains to the southwest---thirteen miles from the coast. Waterways run ribbons through the City (Figure Q.1). Four creeks drain the Gabilan range. These include the Santa Rita, Gabilan, Natividad, and Alisal Creeks. The lower reach of Alisal Creek has been channelized and is known as Reclamation Ditch 1665 (Rec. Ditch). Water from these creeks flow through portions of the City and ultimately empty into Monterey Bay. Together, these water bodies and their watersheds _____________________________________________________________________ Figure Q-1 Regional Surface Hydrology of the Lower Salinas Valley Sub-Watershed Source: 606 Studio Department of Landscape Architecture, California State Polytechnic University, Pomona Vision Plan for Carr Lake Regional Park, June 2003. 278 Q. Watershed Characteristics ________________________________________________________________ form the hydrologic backbone of the Tembladero Slough Watershed. 4 Except for Santa Rita Creek-- see Figure Q-3), each creek is tributary to Carr Lake.5 From their headlands, each of the four creeks makes several passes through farmlands between intermittent trips through urban development. Unlike the rest of the City that is tributary to the Reclamation Ditch, the City’s most southern area drains into the Lower Salinas River Sub-Watershed. Runoff here flows to the Salinas River. Within City limits, the Lower Salinas Sub-Watershed is confined to a relatively small area--2.5 square miles. Outside of the City limits, this is not the case. The Salinas River is the major waterway within Monterey County, and the nation’s longest submerged river.6 It travels 155 miles northwest from San Luis Obispo County through the Salinas Valley into Monterey Bay. The City’s location relative to watersheds as defined by the National Hydrolography Dataset (NHD) is depicted in Figure Q-2. Salinas is shown in the center, in yellow. For the purposes of this report, NHDPlus and other sources were used to create the geographic information systems data layers which form the basis for the City’s assessment and baseline of “watershed characteristics”. The Salinas River and the portion of the City that is outside of the Tembladero Slough Subwatershed, and within the El Toro Creek-Salinas River subwatersheds can be seen in the lower portion of the map. In the larger context, both of these subwatersheds are within the Salinas River Valley Watershed. A closer look at the Tembladero Slough Subwatershed reveals three smaller subwatersheds: 1) Gabilan Creek, 2) Natividad Creek and 3) Santa Rita Creek. Each of these three smaller areas drain to creeks, detention basins and/or other water bodies as shown in Figure Q-3. Carr Lake and Reclamation Ditch 1665 collect flows from these upstream creeks and carry water to the Tembladero Slough. During flood events, Carr Lake serves as a detention basin; otherwise it is heavily cultivated for agriculture. To the northwest of the Tembladero Slough subwatershed lies a small subwatershed (approximately 2 square miles) called Markeley Swamp. Water from this drainage system empties into the Reclamation Ditch northwest of City limits. Farther north, and within the City’s northwestern border, lies Santa Rita Creek. This small creek drains its own minor subwatershed (0.5 square miles) and flows through a small portion of the City before reaching the Reclamation Ditch below City limits. Santa Rita Creek is maintained by Monterey County Water Resources Agency. Within City limits, urban runoff within the Tembladero Slough Subwatershed drains into the Reclamation Ditch and follows a path through the Tembladero Slough, Old Salinas River, Moss Landing Harbor, and the Protrero Tide Gates. The total incorporated area that drains to the Reclamation Ditch system is approximately 13-square miles. Over time, the natural hydrology of the watershed has been substantially altered. The lakes and wetlands that once strung from south of Salinas to what is now Moss Landing Harbor have been 4 Naming conventions of watersheds have been inconsistent. Salinas chose “Tembladero Slough” to be consistent with Federal NHD watershed efforts. Elsewhere, others sometimes call this area the “Gabilan Mountain Watershed”, or “Reclamation Ditch Watershed”. 5 Today Carr Lake is a dry lakebed. It was drained and has been extensively farmed since the turn of the 20th Century. It now serves as a detention basin during flood events. 6 Soil Survey of Monterey County, Natural Resources Conservation Services, 1978. ______________________________________________________________________ 279 Stormwater Management Plan ______________________________________________________________________ drained or altered to make way for agricultural uses and urban development. Figure Q-4 depicts the extent of the area’s former wetlands and 7-lake system that once extended along the Salinas Valley. Like Carr Lake, many of these areas are now under cultivation. 280 Q. 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Watershed Characteristics ________________________________________________________________ Q.3 Flooding “The (Salinas) river tore the edges off the farm lands and washed whole acres down; it toppled barns and houses into itself, to go floating and bobbing away. It trapped cows and pigs and sheep and drowned them in its muddy brown water and carried them to the sea.” --John Steinbeck Stormwater runoff from the Gabilan Mountains poses one of the greatest flood risks to Salinas. Overflows from Salinas River pose a lesser risk due to its distance from City limits. Runoff from the Gabilan Mountains can pass quickly through cultivated farmlands, picking up sediments and exacerbating risks. The flood path from upstream areas draining the Gabilan Range goes through Carr Lake. For decades, Carr Lake has protected Salinas from flooding. However, extreme rainfall events have overtopped lake banks. Carr Lake substantially contains flood events smaller than the 25-year flood. For larger storms this is not the case. In 1998, Salinas experienced a 33-year flood event. Areas immediately surrounding Carr Lake, such as Sherwood Lake Mobile Home Park, experienced flooding. During the 1988 storm, waters breached Highway 101 and Natividad Creek. During a 100-year flood, these areas would also be inundated. Figure Q-5 shows the extent of the 100-year floodplain. The 100-year flood