Ecosystem modeling as a method for designing synthetic fluvial landscapes: a case study of the Salt River in Arizona

291

Landscape and Urban Planning, 20 ( 199 1) 29 l-308 Elsevier Science Publishers B.V., Amsterdam

Ecosystem modeling as a method for designing synthetic fluvial landscapes: a case study of the Salt River in Arizona Edward A. Cook Department of Planning, College ofArchitecture and Environmental Design, Arizona State University, Tempe, AZ 85287-2005, USA (Accepted

14 February 199 I )

ABSTRACT Cook, E.A., 1991. Ecosystem modeling as a method for designing synthetic fluvial landscapes: a case study of the Salt River in Arizona. Landscape Urban P/arm., 20: 291-308. This paper focuses on a method for re-establishing the biological components of a river corridor landscape while accepting many parameters established by the needs of an urban population. The method uses ecosystem modeling to provide a framework for reclamation efforts by patterning a synthetic landscape after natural river corridor landscapes with similar climate, geomorphology and disturbance regimes. The objective is to establish a vital ecological system that can accommodate human activity in appropriate amounts and at proper locations. Application oftechnological and ecological knowledge make this possible. The Salt River corridor case study describes a method in which ecological and multiple-use objectives of an urban population are accommodated.

INTRODUCTION

River corridor landscapes are extremely important in arid regions. In an unspoiled condition, they are among the most highly valued landscapes for wildlife habitat and recreation. A problem has arisen, however, with the association of human settlements and river corridors. In the arid regions of the southwestern United States, the landscape of many river corridors has changed because of the diversion of water for urban and agricultural use (Hastings and Turner, 1965 ). With water removed, leaving nothing to sustain the biological components, the landscape becomes dry and unproductive. In metropolitan Phoenix, Arizona, USA, the Salt River has gone through such a transformation. Once a vital river corridor landscape, it has become dry and barren like the sur0169-2046/91/$03.50

0 199 1 -

rounding desert. With the loss of this type of landscape many important benefits to humans and wildlife are also lost (Johnson, 1985; Petersen et al., 1987; Decamps et al., 1988). Human settlements and vital river corridors need not be exclusive in their existence. The needs of an urban population can still be met, while maintaining the natural qualities of a river corridor. In the case of a largely undisturbed river corridor, human settlements and natural river corridor ecosystems can coexist through conservation strategies which allow for human intervention within prescribed limits (Eastman, 1977; Mann, 1988; Smardon, 1988). In already disturbed river corridor landscapes, such as the lower Salt River, and alternative approach is required. Few studies exist on river corridor reclamation and fewer yet focus on reclaiming dry river corridors in arid regions. The need for such research has been documented by Rea ( 1985), Skinner and Smith

Elsevier Science Publishers B.V.

993 - _

E..4. (‘( )Ok

( 1985) and Morin et al. ( 1989 ) and provides the impetus for this paper. PHYSICAL AND HUMAP q CONTEXT Phjaical

environment

The Salt River corridor is one of the five major river corridors within the state of Arizona. It originates in the White Mountains of EastCentral Arizona approximately 200 km east of the Phoenix metropolitan area just west of the Central Plateau (Fig. 1). Its watershed is approximately 38 850 km’ and includes biotic communities ranging from Spruce-Alpine Fir Forest at the river’s origin to Lower Sonoran Desert scrub on the valley floor where the Salt River joins the Gila River (Lowe, 1964; Brown, 1982 ). The Salt’s watershed includes mountains as high as 3000 m and drops to the desert floor at 400 m above sea level. The climate also varies greatly with January temperatures in the highest areas averaging 1 l- 14°C and in the lowest areas averaging 30°C while in July temperatures at the headwaters average 30 ‘C and in the lower basin 43 oC (Sellers et al., 1985). At the headwaters annual precipitation averages 493 mm and in the lower basin

198 mm (Green and Sellers, 1964). Precipitation is bimodal, occurring in winter and late summer. Aridisols (shallow mineral soils) dominate throughout the basin and approximately 60% of the soils have moderately high to high runoff potential (US Soil Conservation Service, 1975; MacMahon, 1987). This condition contributes to the potential for flash floods with arid region rivers. The transformation of the landscape from the highest points to the basin floor is dramatic. The upper portion of the river corridor (approximately 65 km) is in a predominantly undisturbed state and is very different in character from the lower portion which falls within the Sonoran Desert scrub zone. It is the lower portion which is the focus of this paper. Human impacts

The first evidence of human settlement in the Salt River basin goes back to 300 B.C. when Hohokam Indian settlements existed. The Hohokam used the Salt River as a source of irrigation water, transported through an extensive canal system, to support agricultural production. Later, other Indian cultures migrated into the area and shared the land with the Hoho-

Salt River Basin

Gila

River

Basin

Fig. I. Location of the Salt River watershed with tributaries and Phoenix metropolitan

area.

293

ECOSYSTEM MODELING: SALT RIVER CORRIDOR

Fig. 2. Maps showing the spread of urbanization

along the Salt River from 1925 to 1983.

kam (Ambler, 1977). During the 1870s the first Anglo-American settlements were established in the area. Prior to 1870, the only way to cross the Salt River was on horseback. A ferry crossing was established in 1870 by Charles Hayden at a location that became the nucleus for future development of the City of Tempe (Hayden, 1972). In 1877 and 1878, Mormon parties migrated from Utah and settled Lehi and Mesa townsites (now a part of the city of Mesa), respectively. The Mormons re-excavated abandoned Hohokam canals and re-established use of the Salt River water for irrigation of agricultural land (Merrill, 1970). Following these early settlements the population expanded rapidly creating increased demand for water. This necessitated more formal organization resulting from the enactment of the National Irrigation Act of 1902. The act forced farmers to organize in order to obtain control of water and the Salt River Water User’s Association, a precursor to the present day Salt River Project (SRP), was formed in 1903. During that year, the first dam site on the Salt River was approved and between 1906 and 19 11 the Roosevelt Dam was constructed (SRP, 1984 ) . A total of four additional dams have been constructed on the Salt River and two more on the Verde River since. With the construction of the Granite Reef Dam in 1908, at the confluence of the Salt and Verde Rivers, the water was diverted into a modern day canal system of approximately 300 linear kilometers. The Salt River, below the Granite Reef

Dam, has now been without a permanent water source for over 80 years. Prior to dam construction and water diversion, the Salt River was perennial. Vegetation associated with the river was dense and diverse. Following diversion, the Salt River flowed only during extreme events and periodic, planned releases from upstream dams necessitated by inadequate water storage capacity. Reduced surface flow and increased pumping of groundwater reduced the amount of water available to riparian vegetation. Much of the vegetation could not withstand this condition and has disappeared. This, along with other human-induced manipulations, has caused the shape of the river channel to change over time. Because of the absence of vegetation, which previously contributed to the stability of banks and islands, channel shifts have been accelerated during floods (Dietterick, 1988). Human activity, such as sand and gravel extraction, has also caused significant change. Urbanization adjacent to the river channel and within the floodplain has further added to the transformation. Since the early part of this century, urbanization has spread along the river corridor. Since water flows in the channel infrequently, a more aggressive approach to use of the land within the channel developed. Figure 2 illustrates the spread of urbanization since 1925. An attitude developed among many decision-makers that the river had been conquered. Three dramatic and severe floods, each in excess of loo-year flood levels, in 1978

E.4. COOK

ECOSYSTEM

MODELING:

SALT

RIVER

CORRIDOR

(7363m3s-‘), 1979 (6655m3s-‘),and 1980 (6825 m3 SK’) (US Army Corps of Engineers, 1983) caused extensive damage and loss. In turn, more emphasis was placed on controlling the river and new proposals were developed for channelization. One proposal for transforming the dry Salt River channel became known as the Rio Salado Project. It initially included proposals to turn the riverbed into a 55 km linear park through the entire metropolitan area. This proposal was put before the voters and defeated. The City Council of Tempe, one of the municipalities in the urban area, then voted to proceed with plans to reclaim the segment within Tempe’s jurisdiction (approximately 10 km). The overall goal of the project was to accommodate multiple-use within a recreational theme while addressing issues of flood control, transportation, public recreation, commercial development, organized sport, economic development, environmental quality and wildlife habitat (Fig. 3 ) . RIVER CORRIDOR RECLAMATION Mann ( 1988 ) discusses trends in urban waterfront development. The benefits to cities derived from the renaissance of these areas is well documented (Tsukio et al., 1984). Mann ( 1988 ) also notes increased concern over ecological protection and the danger of additional destruction of wildlife habitat. In the case of the Salt River, severe ecological damage has already occurred within the river corridor. The task then becomes reclaiming it to provide benefits of the urban waterfront and returning ecological values that provide wildlife habitat and sustainability. The goal in the case study examined in this paper was to balance multiple-use objectives of the urban population in creating an urban waterfront and provide a landscape that is sustainable and functional as an ecological system. In developing methodology for this project, it was recognized that re-creation of the Salt

295

River corridor in its previous form (prior to diversion) was not possible nor completely desirable. Many changes have occurred and current conditions would not support complete replication. The objective became creation of a synthetic landscape that accommodates the functions of a natural river regime and uses the elasticity of a balanced ecosystem to accommodate human activity. An approach to design was based upon methods of ecological landscape design espoused by Bradshaw ( 1983 ) . Bradshaw’s four basic approaches to ecological landscape design are laissez-faire, positive construction, manipulation of development and restoration. Laissez-faire allows natural ecosystem development by essentially letting an area ‘go wild’. Positive construction requires deliberate action to create conditions suitable for sustainability in an ecosystem. This is essentially the basis for land reclamation practice (Bradshaw and Chadwick, 1980). Manipulation of development combines the previous two approaches. Bradshaw ( 1983) suggests inoculating the ecosystem with particular ecologically appropriate species and then leaving them to their own devices. Restoration is a combination of the previous three applied to a degraded landscape. In the case of the Salt River, restoration is the guiding philosophy, but with the understanding that the landscape cannot be completely restored to its original form. The initial positive construction, in this case, essentially means configuring a totally synthetic landscape based upon ecological principles that, once established, can be left to its own devices. In the arid southwestern United States there are very few segments of ‘pristine’ river corridors remaining. Because of the historical dependence on water for human existence, these areas have generally been used for settlement, grazing, recreation or other purposes. The lower Salt River has experienced severe modifications as a result of extensive human use.

7%

The Verde River (a tributary of the Salt River) and upper Salt River (Fig. 4) have experienced some modification, although minimal, from human use. The Verde and upper Salt Rivers are perennially flowing and were selected as models of landscape structure to guide landscape planning for reclamation of the lower Salt River. Research using historical photographs and records indicated that the lower Salt River was similar to the existing character of the Verde and upper Salt Rivers, prior to the extensive human-generated impacts that have resulted in the existing river corridor character. The fact that the models for landscape structure (Verde and upper Salt Rivers) do not represent completely natural ecosystems was

EA.

(‘OOk

considered as potential reclamation schemes were studied. Over time, water discharge rates have become regulated and the corridors have been subject to significant human-induced disturbances resulting in increased invader plant species, modified channel character and emigration of some wildlife species. However, the models are still balanced, functioning and sustainable landscapes with demonstrated value for humans and wildlife (Thomas et al., 1979; Johnson, R.R., 1985; Johnson, C.W., 1988) and have similar climate, geomorphology and disturbance regimes (Forman and Godron, 1986; Haase, 1989) to the lower Salt River. Since the lower Salt River exists within an urban context and will be exposed to continued

Fig. 4. Aerial photographs of the Salt (top) and Verde (bottom) Rivers. These rivers were used as models for the design of the synthetic landscape along the lower Salt River corridor in Tempe, .Arirona. The aerial photograph of the Salt River shows the Granite Reef Dam, the point at which all water is diverted into the canal system.

ECOSYSTEM

MODELING:

SALT

RIVER

CORRIDOR

human use, the landscape must be able to withstand continued pressures. The resulting determination was that the modified character of the models was suitable as a guide for design of a synthetic river corridor. Channel configuration andflood control

The genesis of the entire reclamation effort for this project revolves around reintroducing water to the now dry channel. The demands on the water diverted from the lower Salt River corridor have not decreased over the years so an alternative water source is necessary to recreate the river. Two effluent treatment plants will ultimately provide 1 16 million liters of tertiary treated (allows full body contact) water per day. This exceeds the amount required to support the water needs of this project by 100 million liters per day, so the excess will be used for groundwater recharge and sold for commercial use. With a permanent water source secured, reforming the channel and reestablishing vegetation become realistic. Revenue generated through the sale of excess water and newly created waterfront property will be used to partially support public improvements and further reclamation. The need for flood control has been demonstrated with recent flood events (US Army Corps of Engineers, 1983). Management of upstream dams, however, places highest priority on maximum water storage, somewhat limiting flood protection of land adjacent to the river corridor. Rapid or unexpected thaw of snow pack in the White Mountains or major storms may necessitate releases that result in discharges of significant magnitude. The first engineering proposals for controlling flood waters included channelization using a uniform trapezoidal channel cross section (300 m wide and lo- 15 m deep ) and armored banks (Simons, Li and Associates, 1987). Establishment of a channel of this magnitude requires major modification to existing landforms and further excavation of the existing riverbed.

291

The basic engineering parameters of containing flood waters and bank stabilization were previously established and inflexible. However, proposals which lead to a modified version of this channel structure were explored and are explained in this paper. Specific recommendations included multiple level channel structure to accommodate varying discharge rates, burying stabilizing structures. expanded channel width in some locations and more extensive use of vegetation. Analysis of the regimes and physical structure of the Verde and upper Salt Rivers revealed characteristics that were roughly parallel with many of the basic engineering design criteria for channelization of the lower Salt River. The parallel criteria were used as a framework for structuring the design for a synthetic fluvial landscape. Consequently, simulation of the channel forms of the Verde and upper Salt Rivers to the extent feasible, was a method used for accommodating normal fluvial dynamics. Variable levels of discharge are expected within the channel. These range from no dam discharge (normal condition) to the current design flow for loo-year events of 6 100 m3 s- ’ . Improvements to upstream Roosevelt Dam, scheduled for completion by 1996, will reduce the loo-year design flow to 4700 m3 s-’ providing excess capacity for flood control during loo-year events. Flows could be of any magnitude between these extremes, but four levels were established as thresholds for configuring the channel. In the no-discharge condition, only water pumped from the effluent treatment facilities will be present. It will be carried in a braided channel below the elevation established for the main channel (Fig. 5 ). Less than 30 m3 SK’ will flow in a normal situation. The second level is the periodic release of 85-140 m3 s-‘. When upstream dams approach storage capacity, water will be released to preserve capacity for future storms or snow melt. These releases may occur annually during high runoff periods. They reflect the normal cycle of increased flows during runoff periods affecting

298

F.A. ( t )OK

Concrete

Path Level @ 6,100 n?/s Release (100 Year Event)

Level @ 2,700 m3/s Release (10 Year Event) Level @ 140 mj/s Release (Periodi

Bank Stabilizing

Structure

Channel Fig. 5. Generalized

channel

section

showing

the water lcvcls and volumes

the Verde and upper Salt Rivers. At this level, minor inundation occurs in areas adjacent to normal stream channels. The third level is the lo-year flood level. Within the larger channel this would be contained within an interior channel capable of accommodating 2700 m3 S ~ ‘. This quantity of discharge would inundate most of the main channel. Elevation differences between the lo- and loo-year levels are similar to those found along rivers with terraced banks above the main channel. Comparable functions occur in the natural condition during large floods as the process of regeneration is fueled by deposition of fertile soils, seeds and other material. The fourth level is the 1OOyear flood level. Discharge is approximately 6 100 m3 s- ‘, completely inundating the entire channel. Normal flow levels, periodic releases and 1O-year events occurring in the new channel have comparable dynamics and provide reasonable simulations of conditions on the Verde and the upper Salt Rivers, however no precedents exist for significantly larger events with comparable conditions. During floods, movement of material in inundated areas is a normal process. In many situations, structural elements have been used to support banks and keep them from eroding. In the case of the Salt River, because of potential intensity of floods ( IO-year and greater), structural bank stabilization is necessary to maintain the existing channel location and

Bed

at flood stages used as thresholds

for channel

configuratlon.

protect adjacent land uses (Simons, Li and Associates, 1987 ). Structural elements can take many forms (gabions, soil cement, concrete reinforcement and so forth). In the Salt River, the form is not as critical as the fact that they will be covered by soil upon which vegetation will be established (Fig. 5). The soil may be eroded during serious floods, resulting in the need for minor reclamation, but the aesthetic, recreational and wildlife benefits resulting from vegetated banks are sufficient to bear additional costs of periodic reclamation. Concerns about vegetation within a flood channel posing potential problems by uprooting and lodging against structures such as bridge supports resulting in damming of flood waters prevent the use of vegetation in channels engineered for flood control. Large trees usually cause the greatest problems, however grasses, shrubs and immature trees can provide many engineering benefits if used effectively. The most obvious is erosion control. Once established, vegetation will stabilize soil and reduce potential for bank erosion (Schiechtl, 1980). Vegetation also reduces water velocities, one of the primary contributing factors to damage during floods. Many benefits not related to specific flood control issues, such as creating wildlife habitat, improved recreational opportunities, filtering of urban runoff, mitigation of urban heat island effect (Balling and Brazel, I987 ) and others

299

ECOSYSTEM MODELING: SALT RIVER CORRIDOR

can be derived from more extensive use of vegetation. Specific application of vegetation for the Salt River corridor is discussed later in this paper. The design of the channel pattern is reflective of natural stream geomorphology (Cooke and Doornkamp, 1974; Leopold et al., 1979; Castillo et al., 1988). Scouring and deposition are anticipated within the reconstructed channel resulting in changing meander patterns and channel sections. This part of the river’s regime will not be controlled in the same manner that the main channel is, in order to simulate natural conditions. To protect adjacent land uses, structural reinforcement in the form of bank protection was determined to be critical for the lo- and loo-year flood levels to help eliminate migration of the main channel. In some areas, the main channel width is also expanded to vary its character and simulate the form of a natural channel. Only low impact land uses, such as passive recreation, will occur in areas that fall within the loo-year floodplain. Structural support for the banks can be decreased in areas with expanded width because the velocity of flood waters is reduced by the increased channel volume and surface area. Manipulation of landforms within the main channel will create the initial course for the low water condition. Landforms resembling islands, point bars, riffles and pools will be created through the reclamation process and will simulate those found under natural conditions (on the Verde and upper Salt Rivers ) to establish a more stable environment. Two direct benefits can be attributed to this strategy. First, the fluvial processes that generate these forms in a natural condition will act in the synthetic channel once water is reintroduced. These landforms tend to work with fluvial processes making them more stable. Ultimately, this should result in less disruption by minor flood events. Second, the general character of the landscape should reflect that found along a natural river. In an arid region, this is an at-

tractive feature for recreation and wildlife habitat. Island shape, location and composition are the critical elements. In general, islands were designed to be long and narrow (length at least six times width) making them more stable. Islands located between river bends generally reflect natural river conditions. Islands are composed primarily of coarse gravels that are picked up through a bend and deposited soon after when the river straightens out again. Increased stabilization of the island will result with establishment of vegetation. Pont bars are generally located on the interior of a bend and consist of line material (sand, silt and gravel). Deeper pools are created on the outside of a bend because of scouring. Naturalmod&for landscapestructure

Decamps et al. ( 1988) and Morin et al. ( 1989) have recently documented floodplain and riverbank vegetation patterns. The relationship between elevation/topography, sedimentary deposits/soils, depth to groundwater, floodplain dynamics and spatial composition of plant communities is understood. Much is yet to be understood, however, concerning the dynamic nature of floodplain vegetation. In this study, it has been necessary to limit the scope to replicating existing relationships. It is assumed that dynamic and successional processes will be similar to those occurring in the models if the synthetic landscapes effectively replicate ecological conditions. Plant communities used as natural models for this project fall into two main groups, desert and riparian communities (Kearney and Peebles, 1960). Desert communities include creosote bush (Larrea tridentata) and palo Verde ( Cercidium spp. )-saguaro ( Carnegiea giganteu). Riparian communities include the cottonwood (Populus fremontii)-willow (S&x goodingii), mesquite (Prosopis spp. ), saltcedar (Tumarisk ramosissima) mixed and

300

E.A.C'OOK

TABLE Plant adapt

I

communities patterned to greater human use

after those

found

along

the Verde and upper

SEEP WILLOW Trees

Annual Blue palo Verde Cottonwood Mesquite Willow

Cercidiumjlondum Pol)ulus,fremonti Prosop1s spp. Su1i.v goodingri

Shrubs

Salt Rivers

deltordea

Baccharrs

glutinosa

Baccharis

sarothroides

Triangleleaf bur-sage Seep willow Desert broom Brittle bush Tree tobacco Desert tobacco

Encelia,farinosa Nicotiana

glauca

Nrcotrana

trognophj!la

Perrennial

grasses

Cynodon

colonurn

Polypogon

rnonspeliensis

forbs

Hilaria

setaceum

spp.

Thoroghwort

rigida

Punrcum

ohtusum

Penrxsctum

Bowlesra

nuttalllanum

Snapdragon

/nc’anu

Lactuca

serriola

Parreturia,floridana Pectocarya

recurvuta

Polanisiu

trachysperma

Polygonurn Sonchus

Bermuda grass Big galleta Vine mesquite Fountain grass

dactylon

Red brome

ruhcns

Echinochloa

Antrrrhinurn

Ambrosia

or additions

grasses

Bromus

Annual

with some substitutions

lapath!fiJiurn

Clammywced Knotweed

usper

Xunthiurn

succharatum

Cocklebur

.Annuals Eupatorium Euphorhia

aihomarginata

I~elrotropium

Alkali heliotrope Stephanomeria

curussavicum

Stephanomeria

pauclflora

SALTCEDAR Trees

MIXED Annual Blue palo Verde Mesquite

C’ercidium,florrdum

spp.

Prosopis

Shrubs .4mhrosra

spp.

Baccharis

glutlnosa

Baccharis

sarothroides

Encclra.furrnosa

spp.

Isomeris Turnarisk

ramosrssima

MESQUITE BOSQUE WOODLAND (UPPER Trees Prosopis spp. P. chilensrs P. hJ>hnd I’. velutlna Shrubs Raccharis

sarothroide;,

Encelia,farinosu Larrea

tridentata

Zizphus

Annual

ohtusifol,a

TERRACE) Mesquite Chilean mesquite South American hybrid Velvet or Arizona mesquite

arrzonrcus

Rromus

r&ens

Hordeurn

leporinum

Desert broom Brittle bush Creosote Greythorn Arizona brome Red brome Foxtail

forbs

Erodium Sisvmhrium

circutarium Ii70

ari-_onicus

Bromus

rubens

Hordeum

Ieporinum

Qhismus

spp.

CULTIVATED Trees Prosopis spp.

(UPPER

P. alba P. velutina

spp.

C’ercidium

C jloridum

grasses

Bromus

Annual

Bur-sage Seep willow Desert broom Brittle bush Burro weed Saltcedar

grasses

Rrornous

Filaree London

rocket

C’. mlcrophyllum

.Arizona brome Red brome Foxtail Schismus

TERRACE) Mesquite Argentine mesquite Velvet or Arizona mesquite Palo Verde Blue palo Verde Little leaf palo Verde

Shrubs Baccharls

sarothroides

Enceliu

jhrinosa

Larrea

trrdentata

Ziziphus

ohtusili,liu

Desert broom Brittle bush Creosote Grey thorn

to

ECOSYSTEM MODELING: SALT RIVER CORRIDOR

WOODLAND Trees

(LOWER SLOPE)

Prosopis spp.

CULTIVATED Trees Mesquite

Shrubs

Arizona brome Red brome Foxtail Blue grass Schismus

Annual grasses

Coast fiddleneck Filaree London rocket

Annual forbs

Annual forbs Amsinckia intermedia Erodium circutarium Svsimbrium irio

Cercidiumfloridurn

Bur-sage Desert broom Brittle bush Burro weed Creosote

Annual grasses Bromus arizonicus Bromus rubens Hordeum leporinum Poa annua Schismus SQQ.

(LOWER SLOPE)

Prosopis spp.

Shrubs Ambrosia SQQ. Baccharis sarothroides Encelia farinosa Isomeris SQQ. Larrea tridentata

301

seep willow (Bhccharis sarothroides). Since the focus of this paper is the river corridor only riparian communities will be discussed in detail. Plant lists are provided in Table 1. Each of these plant communities is associated with varying topography, soils, groundwater and flood conditions. They are characterized by one or two dominant plant species. Although many of the species are native, notable non-native species, including saltcedar, bermuda grass (Cynodon dactylon) and alifera, are abundant due to previous agricultural and grazing practices. Transition areas or ecotones are also present throughout the areas studied as natural models. Plant species from two or more communities can be found in these areas as the communities overlap in many cases. This is evident most often where elevation changes occur. The location and structure of proposed vegetation within the reclaimed channel is patterned after these natural models. Modifications have, however, been suggested to accommodate more intensive human activity in certain locations. Plant communities documented on the Verde River were mapped as a part of the survey of landscape structure of natural models (Figs. 6 and 7). The methods used for examining landscape structure in-

Ambrosia spp. Baccharis sarothroides Encelia farinosa Isomeris SQQ. Larrea tridentata Bromus arizonicus Bromus rubens Hordeum leporinum Poa annua Schismus SQQ. Amsinckia intermedia Erodium circutarium Sysimbrium irio

Blue palo Verde Mesquite Bur-sage Desert broom Brittle bush Burro weed Creosote Arizona brome Red brome Foxtail Blue grass Schismus Coast fiddleneck Filaree London rocket

eluded study of existing published data and field investigation. Aerial photographs were used to prepare preliminary maps of locations of vegetative communities. Field investigations were conducted to verify mapping done from aerial photographs. In each mapping unit, sample areas of approximately 30 m* were investigated in more detail. Species distribution and sizes were documented to provide an understanding of the internal structure of the mapping units. Data were also collected concerning location of plant communities within the river corridor using transects. Through this method, important relationships between vegetation and bank or terrace elevations became evident. A generalized corridor section of the Verde River illustrates this configuration (Fig. 7 ) . Published soils data (Broderick, 1979 ) and additional field investigations provided further understanding of the relationships between soils, flood regimes, landforms and vegetation. The design for the reclaimed channel (Figs. 7 and 8) follows that of the natural model but is slightly modified, in some cases, to accommodate development and recreational interests. Although the data collected concerning landscape structure of the models were specific to certain locations, generalized patterns

302

EA.

COOK

303

ECOSYSTEM MODELING: SALT RIVER CORRIDOR

Verde

Synthetic

River

River

Corridor

Corridor

Section

Section

Fin. 7. Generalized cross sections of the Verde River (top) and the design for the reclaimed lower Salt River (bottom) the location of plant communities within the channel.

could be discerned and provided the overall structure for the initial design for the reclaimed river corridor. In time, as various natural processes begin to work on the designed or synthetic river corridor landscape, the complexity of the mosaic will increase and reflect the dynamic state of a vital landscape. Gravel and sand bars located in the center of the channel (islands) and along the edge of the active channel are dominated by seep willow and saltcedar mixed plant communities (Fig. 9). Lower areas and relatively unstable sand bars are generally dominated by seep willow. These areas are frequently flooded (subject to upstream dam releases) and experience considerable fluctuations in water level. Consequently, the plants found here have ample water but may be lost or disrupted by floods and remain fairly small. Groundwater is available to plant root zones and soils are generally gravels and fine deposits. The saltcedar mixed community is found in slightly higher areas within the channel. These areas are generally larger and more stable islands and point bars. Because they are more stable, some larger plants are found here since the areas are dis-

indicating

rupted less frequently by floods. They would be completely inundated during 1O-year floods and because of periodic disturbances plants may be displaced or damaged prior to reaching full size. Occasional patches of older cottonwoods can be found along both the Verde and upper Salt Rivers. They are generally located on slightly higher terraces near the edges of the channel in finer soils. Most of the trees are over 50 years old, suggesting that since flow regulation and overgrazing have occurred, conditions suitable for reproduction have not been present (US Bureau of Reclamation, 1976). The cottonwood-willow plant community was not incorporated into proposals for the reclaimed channel of the Salt River for two primary reasons. First, the condition of existing patches suggests that because of regulation and subsequent changes in the natural regime of the river, conditions are no longer suitable to support reproduction. Floods, disease and human impact may further deplete the existing stands in the near future. Second, large cottonwoods, uprooted in floods, have been responsible for collecting debris and creating blockages which

Ezl Ornamental m

Pa10

Verde

Salt Cedar _ Ironwood

m

Palo Verde

Fig. 8. Plan view of a segment of the reclaimed lower Salt River improvements and the re-established water course.

have caused significant property damage in previous years. Cottonwoods are more prone to this than mesquites because they are generally located nearer to the edge of the main channel and at lower elevations where water velocities are greater and floods are more frequent, increasing potential for uprooting or damage. Cottonwoods are also a fast-growing, weakly structured trees so the potential for breakage of large limbs is high. Some cottonwoods may become established through natural regeneration processes and could remain. If they reach excessive size and become a potential hazard, removal may be considered. Mesquites are found mixed occasionally within the saltcedar community and also in

IImIll Mesquite - Saguaro showing

location

Seep Willow of plant

communities

in relation

to physical

bosques that occur on higher terraces farther from the active channel. The mesquite bosques are part of the Sonoran Riparian Deciduous Forest and Woodland Formation (Brown, 1982). Mesquite bosques form a dense canopy and provide some of the most valued wildlife habitat in Arizona (Gavin, 1973; Turner. 1983). They occupy gently sloping terraces comprised of coarse to sandy soils with high potential for erosion in extreme runoff conditions (Broderick, 1979 ). Varying conditions analyzed in natural bosques lead to the formation of two general categories: the woodland mesquite bosque (Fig. 10 ) and cultivated mesquite bosque (Fig. 11). The varied characteristics of these two types of bosques are

ECOSYSTEM

MODELING:

SALT

RIVER

305

CORRIDOR

Fig. 9. Photographs of gravel and sand bars dominated by seep willow and saltcedar mixed plant communities

Fig 10. Photograph of a mesquite bosque along the Verde River after which the synthetic woodland mesquite bosque is patte :rned.

regarding apuse :ful for making distinctions bpriate use. prc 1The woodland mesquite bosque includes two

sub-categories: upper terrace and lower sic3pe. This zone is found adjacent to rivers an d is comprised of regularly spaced (average 5 m)

E.A. (‘OOK

306

Fig.

I I. Photograph

ofa mesquite

bosque

along the Verde River after which the synthetic

smaller mesquite (multi-stem) trees with associated shrub and understory plants. This zone is well suited for wildlife habitat. The upper terrace is slightly higher than the lower river channel and is in an upper floodplain (within loo-year and above 1O-year ). The lower slope is a transition zone that includes a lower tree density and drops down to the river level. This structure is found in largely undisturbed natural mesquite bosques. Mesquite bosques are flooded periodically, but lower water velocities outside the main channel result in minimal loss or damage. The cultivated mesquite bosque is similar to the woodland mesquite bosque, except that trees are spaced further apart and become larger. An upper terrace and lower slope are also present in this area, however trees in the lower slope area are smaller and more dispersed than those on the terrace. In the upper terrace, trees provide a high canopy and much needed shade. It is well suited to more intensive recreational activities such as picnicking. Tree spacing (8-10 m) allows free movement

cultivated

mesquite

bosque

is patterned.

on the ground. In natural situations, similar landscape structure was found in mesquite bosques intensively used for unstructured recreation over a number of years (Turner, 1983). CONCLUSION The principal value of a process, such as described in the case study of the Salt River, is demonstrating that integrated urban and natural systems can provide measurable benefits to the urban population and also improve the state of a disturbed environment. As competition intensilies for land within an urban setting, ecological landscapes must be capable of assuming multiple uses. Conversely, landscapes that have been developed traditionally with an urban systems orientation must assume more ecological functions. It will always be appropriate to preserve biotopes that are important sanctuaries for wildlife, but through urban reclamation processes networks can be rebuilt that will contribute in a more meaningful way to making natural systems in the city whole again.

ECOSYSTEM

MODELING:

SALT

RIVER

307

CORRIDOR

Bradshaw ( 1983) notes that an essential characteristic of ecological design is to understand what nature achieves and simulate natural conditions to develop harmonious, integrated ecosystems, He also notes an essential paradigm of nature is what ultimately survives is that which is fitted best. Therefore, if plans are patterned after natural systems, with sufficient understanding, the solution will be less likely to be rejected by nature. This important principle can also provide economic justification often required to implement any proposal. The economy of integration of urban and natural systems can be clearly demonstrated through the concept of multiple use. Most will agree that healthy ecosystems are important to the general welfare of society, but with specific projects measurable benefits to the urban economy must also be demonstrated if implementation is the goal. The conditions under which this study was undertaken were unique. Similar issues exist, however, in many settings and the principles of ecosystem modeling may be equally appropriate as a method to reach a solution in other circumstances. Historically, incompatibility between natural and urban systems resulted because of a lack of knowledge about the flexibility of natural ecosystems and the adaptability of urban systems. Lines between natural and man-made are becoming less distinct as advances in biotechnology, ecology and various other natural sciences contribute to the body of knowledge that can be applied to environmental problems. The result should be increased opportunities to pursue projects of a similar nature. ACKNOWLEDGMENTS The author expresses his gratitude to the anonymous reviewers for insightful suggestions that helped form the final version of this paper. Thanks are also due to John Pennell Phillips and William Whitmore for assistance with graphics and project planning. Initial

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