Orange County California Watershed

2.1 Problem Statement

The Aliso Creek watershed suffers from a number of problems related to water resources. Not all of these problems are human-induced. Some of these problems are only the response of a natural system to variation in rainfall, sediment fluxes, and other occurrences that are part of the evolution of a watershed. Some perceived problems, such as erosion and flooding are actually natural processes within any watershed. However, human actions and land uses can magnify the scale of these problems, particularly downstream. This is not to say, however, that no actions should be undertaken to correct problems in the Aliso Creek watershed. For example, where human infrastructure has been placed in areas naturally prone to erosion and flooding, they can suffer damages and may require protection. It may also be beneficial to protect remaining habitats in order to prevent localized extinction of species.

The list of problems currently evident in the Aliso Creek watershed is certainly not 100 percent complete, but contains the most significant factors that were agreed upon by the individuals, agencies, and other parties that participated in its development. A number of public meetings and workshops, study team meetings (stakeholders), and site investigations were held to develop this list. The identified problems are grouped in four general categories: creek instability, water quality, loss of fish and wildlife habitat, and flooding damages (Table 1). Specific problems within these categories are briefly discussed in the following sections.

The problem of channel instability is regarded as one of the most fundamental problems in the Aliso Creek watershed, which affects other identified problems. This problem has been related to natural channel change, development inside the watershed, increased flood flow peaks and/or volumes, increased dry weather (low-flow) discharge, impervious surface runoff increases, the random nature of recent large flood events, and other issues. Whatever the reasons, degradation (lowering) of the channel invert, which historically would be interspersed with periods of channel aggradation (or infilling), has turned into an increasingly destructive trend as the cyclical erosion and fill cycle has been replaced by continued degradation. It can be seen that replacement of bare soils in the watershed by development has cut off the traditional source of sediment in the watershed. This being the case, it may be that the now hungry or sediment-poor runoff from the watershed is compensating by picking up more of its characteristic sediment load from the channel bed itself. Ultimately, a lack of sediment as a source will result in continued erosion in other locations, and eventually a lower sediment delivery to the coast. This will have long-term negative effects on beaches downcoast, as sooner or later, the channel source will also be exhausted, robbing the beaches of needed sediment.

Erosion by surface water flow is currently causing land loss to adjacent properties. This is largely due to degradation of the channel (channel instability), which has been increasing since the late 1960s. Although this has been related to development of the watershed, increased impermeability, and increases in flood flow peaks and volumes, there is no definitive cause-and-effect relationship. It is sufficient to recognize that erosion of channel bed and banks is increasing and that land loss is accelerating. It is also recognized that treatment of the existing channel instability problem may reduce, or in some cases halt land loss by erosive forces. Because there is less damage attributable to land loss than that of overall environmental degradation, treatment of this problem is viewed as being only an incidental benefit of a larger environmental restoration campaign. Therefore, this problem will be discussed and evaluated as part of the larger watershed problem of channel instability and related environmental degradation. See Photo 2 at the top of this page.

2.2.2 Loss of Floodplain Habitat

Floodplain habitat, as discussed here, refers to vegetation complexes that would be found within the floodplain, or overflow area from most flood events.

The gradual conversion of floodplain habitat from trees and bushes of certain more water-dependent types, dominated by the complex known as "California Oak Woodland," to those of a more xeric (drought tolerant) nature is related by many observers who have spent much time in the watershed. Several long-term residents have noted that the trees once found in the floodplain are now largely gone, replaced by scrub and dry grasses. It is believed that many trees were cut in the "Mission" period, as the oaks, sycamores, and other species were a valuable resource in the production of ships, structures, charcoal, and other uses. Still, many trees survived into this century, as evidenced by in-person accounts with older residents. It is not definitively known what caused the recent disappearance of trees in the floodplain, particularly in the lower watershed, but it has been noted that areas dominated by channel degradation have few resident trees, and those not suffering from appreciable degradation have a much greater associated tree population. Given that the degradation of the channel has been accompanied by a decline in floodplain soil moisture levels, it may be the case that the source of water for these large trees has disappeared and taken the trees with them. Tap roots for these trees, although lengthy, may have been of insufficient length to reach the far deeper groundwater table under current conditions.

Environmental resource agencies, land managers, and wildlife specialists have indicated that historic floodplain vegetation is rapidly disappearing in southern California. Given that much of the lower reaches of Aliso Creek are currently in public stewardship (i.e., Aliso and Wood Canyons Regional Park), this may provide a unique opportunity for restoration of the historic floodplain vegetation complex, and the wildlife dependent on it.

This issue shares similar factors to that of floodplain habitat. Much of the riparian habitat in the Aliso Creek watershed has also suffered the same kind of destruction as that on the floodplains. Some regrowth is evident in some reaches, but tends to disappear with each flood event. This could be lessened if the structure of the stream channel were more stable. Riparian habitat, which supports fish, reptiles, insects, and mammals that traditionally occupied the watershed, is more evident in the Wood Canyon sub-watershed, and within some of the upper reaches of Aliso Creek. Since this habitat is dependent on both water availability and structural stability, much of the success of a riparian environmental restoration campaign is dependent on the success of channel restabilization measures.

Non-native (exotic) species are those not naturally found in a given area but through some transport mechanism have successfully occupied a biological niche. Often, these species have the ability to outcompete native species through specialized adaptations or an absence of predators. The increase of non-native species negatively impacts riparian ecosystems by decreasing the diversity of native habitat and frequently forming dense monocultures.

The giant reed (Arundo donax) is the primary exotic species to invade the Aliso Creek system. Giant reed is a hydrophyte, growing along lakes, streams, drains, and other wet sites. It uses prodigious amounts of water to supply its incredible rate of growth. Under optimal conditions, it can grow more than three inches per day (TNC, 2000).

This species is well adapted to the high disturbance dynamics of riparian systems as it spreads primarily vegetatively. Flood events break up clumps of Arundo and spread the pieces downstream. Fragmented stem nodes and rhizomes can then take root and establish as new plant clones.

Establishment and success of giant reed within a riparian corridor thus results in a decline in the diversity of native riparian plant species. All evidence indicates that giant reed does not provide either food or habitat for native species of wildlife. Areas largely taken over by this species are therefore deprived of wildlife.

Two other non-native invasive plants that are reported to be in the lower Aliso Creek riparian zones are the castor bean (Ricinus communis) and purple thistle, which is most likely bull thistle (Cirsium vulgare). A more complete list of non-native species found in Aliso Creek is given in Table 2.

Photo 4: Contaminated water sign at Aliso Beach

The size and natural diversity of the landscape in the Aliso Creek watershed produces numerous opportunities for public recreation, education, and environmental awareness. Recreational activities available throughout the watershed include bird watching, fishing, hiking, jogging, surfing, golfing, and mountain biking. Many recreational parks and facilities are linked to a local watercourse within the watershed. In fact, some of these parks and facilities depend on Aliso Creek, or one of its tributaries.

In the watershed study area, residential development has increased at a rapid pace, making the existing recreation parks and facilities even more valuable for humans, as well as pockets of wildlife habitat. Orange County conducted a recreation analysis that examined opportunities throughout the watershed, identifying two major areas for detailed analysis, Aliso and Wood Canyons Regional Park and the Aliso Beach Park. These areas were selected because they have a history of lost recreation opportunities tied to Aliso Creek's poor water quality problems. As recreational opportunities are lost due to creek instability, watershed management practices become critical for preserving some of the area's parks and facilities.

While Aliso Creek channel instability is most pronounced in the lower reaches, poor water quality is a pervasive problem throughout the entire watershed. Surface water quality in Aliso Creek has been in a state of decline since intense development of the watershed began in the 1960s.

The Aliso Creek watershed has been designated by the San Diego Regional Water Quality Control Board (SDRWQCB) as a target watershed for priority water quality enhancement efforts. Aliso Creek is listed as a Category I Impaired Priority Watershed (Aliso-San Onofre, #18070301) in the California Unified Watershed Assessment List (USEPA, 2000). The section of the creek from Aliso Beach to one mile upstream is designated as impaired for high coliform concentrations under the 1998 Clean Water Act Section 303(d). The primary causes of impairment of this watershed are non-point source pollution. Residential and commercial use of fertilizers and pesticides, and pet and waterfowl waste, are most likely the primary contributors to the nutrient and potential stormwater toxic impacts and elevated bacteria load. High temperatures also contribute to poor water quality.

Water temperature is a crucial factor in stream restoration for the following reasons:

Recent surveys of Aliso Creek indicate a lower diversity and abundance of aquatic wildlife than is recorded in historical accounts. Small fish and some aquatic insects like the dragonfly (Macromia sp.) and the non-native mosquito fish (Gambusia affinis) still inhabit the creek's waters, but the effects of aquatic and riparian habitat degradation are clearly evident. In addition, flood events have severely impacted aquatic wildlife. For example, following the 1983 flood all remaining large fish including the bluegill (Lepomis macrochirus) and non-native bass (Micropterus spp.), and large indigenous frogs totally disappeared from the watershed (PFRD, 2000).

The degradation and loss of formerly stable riparian and floodplain areas combined with exceptional natural events have caused the elimination of critical aquatic species from the waters of Aliso Creek. Historical accounts indicate that steelhead trout may have been present in Aliso Creek until the late 1960s or early 1970s (USACE, 1997). In 1999, an aquatic life assessment was conducted by Orange County to provide a qualitative and quantitative evaluation of the existing aquatic community in Aliso Creek. Six benchmark sites, listed below, were identified and sampled.

These sites were chosen for their ability to represent the typical range of conditions found within the Aliso Creek channel. Of these sites, Upper Sulphur Creek station exhibited the poorest habitat due to the large accumulation of organic sludge in the substrate generated by excessive algal growth. In general, most organisms still present in the creek were tolerant of degraded conditions and further underscore the fact that the water quality conditions in the watershed are poor.

This problem is defined as the inundation of structures and other valuable property by floodwaters in such a way that monetary damage is caused. It is important to understand that ANY structure in the watershed could be inundated by water at any time. Water entering a home or business may not come from one of the obvious channels in the area, but might come from a slope upstream, a backed-up storm drain, or a burst water main. It is also in the nature of rainfall in southern California that a high-intensity storm cell may "park" itself over a neighborhood and exceed the ability of local drains to carry the runoff away. Repair of damages caused by these events is not normally covered by homeowner's insurance. This is why any homeowner, even those far removed from an obvious storm drain or channel would benefit from obtaining a flood insurance policy. Currently, there are not many sites within the Aliso Creek watershed that are at a significant risk of flood inundation.

The Aliso Creek Watershed Management Feasibility Study (USACE, 1999) included an analysis of the current flood threat in the Aliso Creek watershed. The results indicated that most structures in the watershed, and all continuously occupied residences, have a very low probability of flood inundation at this time. Several schools and at least one church are currently at the margins of the "100-year" floodplain. The 100-year floodplain encompasses an area in which the risk of inundation is, on average, one percent in any given year. This means that this size flood event is rare and should only occur, on average, approximately once in a 100-year period. It does not mean that this size flood cannot occur several times in a century, or even twice in one year. The given frequency is not a guarantee, but an estimate based on limited knowledge of past flood events. The schools and church are not currently at a high risk of inundation. It would take a major flood to threaten these institutions. During such events, it would be wise for staff from these facilities to monitor water height in the channel nearby and have a plan in place to evacuate to higher ground should water go over the banks. It does not, however, make economic sense to floodproof these structures at this time due to the high cost involved and low probability of inundation.

Photo 5: Debris on Aliso Creek InnGolf Course after 1997 - 1998 floods.

The single most vulnerable site in the Aliso Creek watershed is that of the Aliso Creek Inn and Resort in the canyon mouth and a small number of properties in close proximity. This site has been determined to be within a "25-year" floodplain, or to have a risk of inundation of, on average, approximately four percent in any given year. The Aliso Creek Inn and Resort has suffered significant damage during moderate to large flood events. The sites in the canyon mouth have been at risk from flood inundation since prior to development of the Flood Information Study generated by the Corps of Engineers and the County in 1973.