Hydrologic characteristics of a small desert mountain stream: implications for short-term magnitude and frequency of bedload transport

Journalof Arid Emnronments (1990) 18,151-163

Hydrologic characteristics of a small desert mountain stream: implications for short-term magnitude and frequency of bedload transport Bruce L. Rhoads* Accepted 6 March 1989 This paper examines the magnitude and frequency of fluvial processes over a 24-year period for a small desert mountain stream in southern Arizona. The hydrologic data indicate that flow occurred in this channel only 0'05% of the time. The probability density function of flow is distinctly non-lognormal and approximates a one-parameter gamma distribution due to the lack of base flow. Estimation of bedload transport for the flows indicates that the effective discharge for transport is probably equalled or exceeded only about 0'01 % of the time, but may have a recurrence interval of approximately 2·5 years. A close correspondence between estimated bankfull flow of the active channel and the effective discharge suggests that equilibrium between form and process occurs in this system. However, because the active channel lies within an arroyo, other factors, such as catastrophic floods, appear to shape the gross channel morphology, which is in disequilibrium with the short-term hydrologic regime.

Introduction

Although mountains are common features in many drylands of the world, little is known about the magnitude and frequency of fluvial processes in desert mountain streams. This lack of knowledge primarily reflects the difficulties associated with collecting hydrologic data in arid environments (Schick, 1978). The infrequency and short duration of runoff creates enormous logistical problems for direct measurements of flow characteristics. Expensive automatic measuring and recording devices represent the only plausible methods for obtaining such data. The magnitude and frequency of flash floods in arid uplands are important practical concerns for urban development of adjacent piedmonts (Rhoads, 1986a). These factors are also significant geomorphically because they are related directly to explanations of channel form. Numerous studies have demonstrated that flows of moderate magnitude and frequency transport the largest fraction of suspended sediment and bedload for rivers in humid temperate environments (Wolman & Miller, 1960; Pickup & Warner, 1976; Fisk, 1977; Andrews, 1980; Webb & Walling, 1982; Ashmore & Day, 1988). However, much less is known about the magnitude and frequency characteristics of dryland rivers. Baker (1977, Fig. 3) argued that two factors associated with increasing aridity should decrease the frequency and increase the magnitude of the effective discharge for sediment transport. First, as climate becomes drier flow variability tends to increase. This variability is expressed as strong positive skewness in the lognormal probability density • Department of Geograph y, University of Illinois, Urbana, IL 61801, U.S.A . 0140-1963/90/020151 + 13 $03'00/0

© 1990 Academic Press Limited

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function of streamflow, causing a larger percentage of the total stream load to be carried by infrequent flows. In addition, the coarse bed material that commonly occurs in dry land rivers decreases the frequency and increases the magnitude of the effective discharge by increasing the threshold for sediment transport. The relationship between the effective discharge and channel morphology is still uncertain. Wolman & Miller (1960) suggested that the effective flow is roughly equivalent to bankfull or channel-forming discharge. This thesis was corroborated by Andrews (1980). Other studies have indicated that the effective discharge occurs more frequently than bankfull flow (Carlston, 1965; Benson & Thomas, 1966; Nolan et al., 1987). Pickup & Warner (1976) also found that the effective discharge occurs more frequently than the bankfull event and suggested that there are two groups of morphologically-significant events: moderate flows that shape the bed and large flows that shape the banks. Recently, Carling (1988) used short-term data on sediment transport rates to determine the effective flowfor two gravel-bed streams in northern England. He found that the effective discharge for bedload transport could be equated with the bankfull (dominant) discharge for the stream with easily-erodible boundaries, but the bankfull discharge and the effective flow were not equal in the channel with constrained boundaries. A different view of geomorphic effectiveness was proposed by Wolman & Gerson (1978). They asserted that change in the shape of landforms, rather than the amount of sediment transported, is the true measure of effectiveness. This perspective emphasizes the timing of events, the role of major floods in shaping rivers and floodplains, and the rate of recovery of a river to an erosional event (e.g. Beven, 1981; Kochel, 1988). It also casts doubt on the concept of a dominant discharge, i.e. that an event of a given recurrence interval can represent the range of morphologically significant flows (Richards, 1982). Several investigators have proposed that the average period required for a dryland river to recover its prior form may exceed the mean recurrence interval of large, highly erosive floods (Stevens etal., 1975; Burnsden & Thornes, 1979; Kochel, 1988; Graf, 1988). Under these conditions equilibrium between channel form and a discharge of moderate magnitude and frequency is not possible. Although evidence from large arid-region rivers tends to support this hypothesis (Burkham, 1972; Graf, 1983), it has yet to be examined for small ephemeral streams. The purpose of this paper is to analyze a 24-year hydrologic record for a small desert mountain stream in southern Arizona to (1) determine the magnitude and frequency of flow during this period, (2) estimate the magnitude and frequency of bedload transport, and (3) explore the relationship between the modern channel morphology and the magnitude and frequency of sediment transport. Location and environmental setting The ephemeral stream examined in this study is a small unnamed channel that drains a 4·5 km2 watershed in South Mountain, an east-west trending mountain range near Phoenix, AZ (Fig. 1). Vegetation cover in the basin is sparse and consists of several varieties of small trees (paloverde, mesquite, ironwood), shrubs (creosotebush, ocotillo, bursage), and cacti (giant saguaro, cholla, prickly pear) (Turner, 1974). The climate is arid subtropical with a mean annual rainfall of about 200 mm and a mean annual temperature of 20°C(National Oceanic and Atmospheric Administration, 1983). The topography of the watershed is fairly rugged with a maximum local relief of about 400 m (Fig. 1). Tributary channels are steep, having slopes as high as 20% near the drainage divide. Granite and gneiss are exposed on the beds of headwater tributaries, which are confined within narrow ravines; lower reaches are incised discontinuously into late Quaternary alluvium (Reynolds, 1985). The depth of incision, which ranges from 0·5 to 4'0 m, varies significantly over short distances. Near the mouth of the basin, the active channel of the main stream is 8'8 m wide with a

HYDROLOGIC CHARACTERISTICS OF A SMALL DESERT MOUNTAIN STREAM

153

Contour loterval. SOm

o ARIZONA

km

10

Figure 1. Location of the study site. mean depth of 0'13 m, the median grain size of the bed material is 2'8 mm, and the local channel gradient is 0'025 m/m (Osterkamp et al., 1982; Rhoads, 1986b). The active channel is defined by 'a break in the relatively steep bank slope of the active channel to a more gently sloping surface beyond the channel edge' (Hedman & Osterkamp, 1982). This break in slope also coincides with the lower limit of permanent riparian vegetation. Alluvial gravels that have a fresh, unweathered appearance occur between the active channel banks.

Hydrologic data The U.S. Geological Survey has operated a continuous water-stage recorder at the South Mountain site since January 1961 (Salt River tributary in South Mountain Park, gaging station # 095122). Daily information for this gaging site is published in the U.S.G.S. Water-Supply Paper series (1961-1965) and in the Water Resources Data for Arizona publications (1966-1984). These data constitute one of the longest continuous hydrologic records available anywhere in the world for a small desert mountain stream. Control at the site is good for all but the largest discharges since stage is measured as flow passes through a closed conduit beneath a road crossing. Peak flows for large events are generally computed using the slope-area method. In addition to the published information, the original strip charts from the water-stage recorder were examined to extract data on total flow duration (D) and the duration of the rising limb (Tp) for each event as well as the magnitudes of peak discharges below 0·57 m3/s (the base discharge for the site).

Hydrologic regime of the South Mountain Basin Between January 1961 and October 1984 a total of 45 separate flows were recorded at the South Mountain gaging site (Table 1). The total duration of these flowswas approximately 99'25 h or 4·1 days. Although the water level recorder was not operational for 293 days overthe 8674 day period of record, U.S.G.S. personnel estimated that no flow took place on these days based on the lack of field evidence of flow, precipitation records for nearby weather stations, and a crest-stage recorder at the site (Francis Jelinek, pers. comm.). Thus, the data suggest that water flowed in this channel less than 0'05% of the time.

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154

Table 1. Floodevents recorded at theSouth Mountain gaging site Duration (min)

Date 2 July 1961 22 July 1961 19 Oct. 1963 14 July 1964 1 Aug. 1964 2 Aug. 1964 26 Aug. 1964 13-14 Sept. 1964 23 June 1965 8 Sept. 1965 10 Dec. 1965 22 Dec. 1965 17 Aug. 1966 18 Aug. 1966 3 Sept. 1967 12 Feb. 1968 30 July 1968 4 Aug. 1970 18 Aug. 1970 5 Sept. 1970 22 June 1972 12 Aug. 1972 6 Oct. 1972 19 Oct. 1972 19 Oct. 1972 11 Nov. 1972 17 Nov. 1972 28 Dec. 1972 20 Mar. 1974 31 July 1974 28 Oct. 1974 29 Oct. 1974 22 Sept. 1976 23 Sept. 1976 23 Oct. 1976 29 Dec. 1977 19 Jan. 1978 5 Mar. 1978 1 Oct. 1981 13 Mar. 1982 30 Nov. 1982 3 Mar. 1983 27-28 July 1984 1 Sept. 1984 10 Sept. 1984

Time to peak (min)

0·11 10'48 15·00 0'79 3·40 9'40 4'56 0·93 0·96 18'97 0'48 0·25 2·38 5-49 0'34 0-40 2-29 0'06 0'15 0·25 0'04 0'06 1'76 2·43 2-26 4'70 1·53 0'14 3'22 0·08 0'08 0·10 1·59 1'98 0·14 2'43 0.59 0'45 3'77 0'03 1·25 0-37 18-23

30 100 120 85 90 140 240 40 60 180 50 75 100 180 60 60 150 40 50 95 30 40 210 180 180 165 190 100 300 40

7·5 20 20 20 30

120 120 280 220 205 120 85

5

4-28

70

5·71

5

135 35 115 75 240

135

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15

5 5

20 5

5

55

5

12'5 15 25

5

20 5

5 5

10

5

5 5 5

7·5

10 25 20 30 25 12'5

Two-thirds of the runoff events and all of the largest peak flows occurred between July and October (Table 1). This season is often referred to as the Arizona summer monsoon (Carleton, 1985). It is the time of year when convective thunderstorms are most common in southern Arizona (Bryson & Lowry, 1955). The intense rainfall associated with these storms is the primary flood-generation mechanism for small watersheds in the southwestern United States (Osborn, 1983). Only minor peaks occurred from November through

HYDROLOGIC CHARACTERISTICS OF A SMALL DESERT MOUNTAIN STREAM

155

March (Table 1). Precipitation associated with winter frontal systems, which can produce large floods on major rivers in central Arizona (Graf, 1983), usually is not intense enough to generate large volumes of runoff in small drainage basins in the Southwest (Osborn, 1983). No flows were recorded during the months of April and May over the 24-year period. The spring months in southern Arizona are relatively dry. The precipitation that does occur is usually of low intensity and thunderstorms are rare (Schmidli, 1978). The average interflood interval during the period of record was 192 days. This value is less than the average waiting period of 365 days for floods in N ahael Yael, a small mountain catchment in the hyperarid Negev Desert of southern Israel (Schick, 1988). The standard deviation of the interflood interval was 281 days, indicating that the time between flow events is extremely variable. The maximum interval was 1305 days; no flows were recorded at the site between 5 March 1978 and 1 October 1981. It is interesting to note that during this period several large floods occurred on major rivers in the Phoenix Basin (Graf, 1981; 1983). Flow frequency The standard method for evaluating flow frequency involves dividing the range of mean daily discharges into small classes and tabulating the number of days within each class (Wolman & Miller, 1960; Pickup & Warner, 1976; Andrews, 1980; Ashmore & Day, 1988). Such an approach is acceptable for large perennial streams that generally have broad flood peaks and minimal diurnal flow variation. However, it is inappropriate for an ephemeral stream like the one examined in this study in which flowslast only a few hours at most. In this situation, frequency analysis based on mean daily values would exclude most of the large flows. Thus, an alternative method of analysis was adopted for this study. Examination of the strip charts for the water stage recorder at the South Mountain site revealed that almost all of the hydrographs could be accurately represented by a simple triangular form whereby the apex of the triangle is the peak stage and the two other corners are times of zero flow (Fig. 2). Triangular hydrographs were constructed for each of the 45 flow events recorded at the South Mountain gaging site using the information on peak 1'50

.§

-r------,

1·00

0·50

Figure 2. Pen tracingfrom waterstagerecorderstrip chan for eventof 17August 1966 illustrating

triangular form of the stagehydrograph.

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156

stage, duration of the rising limb, and total flow duration derived from the strip charts, The hydrographs for a few of the events were distinctly more complex than a simple triangle and multiple stacked triangles were used to represent these shapes accurately. The total range of flows was subdivided into 38 classes at 0'S-m 3/s intervals. The duration of flow within each class (D)) was computed trigonometrically using the formula: D) = (GH u

-

GH1)/tan e

where GH u is the gage height for the discharge at upper limit of the class interval, GH) is the gage height for the discharge at the lower limit of the class interval, and tan e equals GHJfr/d where GH p is the peak gage height and T r/d is the duration of either the rising limb or falling limb. This method differentiates the continuous hydrograph of instantaneous flow for each event, rather than the hydrograph of mean daily values. Thus, flow durations for each discharge class can be expressed in minutes rather than days. The flow frequency histogram exhibits a shape that is distinctly non-lognormal (Fig. 3). Small, low-frequency discharges are absent due to the lack of base flow. The probability density function appears to approximate a one parameter gamma distribution rather than a lognormal one (Yevyevich, 1972). Alternatively, one might view this distribution as a lognormal distribution that is skewed so strongly to the right that the mode has shifted to the origin. The frequency of floods for the South Mountain gaging site was analyzed using the logPearson Type III method (U.S. Water Resources Council, 1981) (Fig. 4). A conditional probability adjustment was made to correct for zero-flow years (Jennings & Benson, 1969). To assess the relative size of the events that occurred during the 24-year period, Roeske's (1978) flood frequency equations for Region 2 (Southwest Desert Area) of Arizona were used to compute magnitudes of floods for various recurrence intervals for the South Mountain site (Fig. 4). The predicted and computed flood frequency curves are quite similar for low frequency events, but Roeske's equations tend to overpredict the magnitudes of small floods. The predicted value of the 2S-year flood is 21·78 m 3/s with a standard error of +6S%, whereas the flood of record over the 24-year period is 18·97 m 3/s. Thus, the largest event does not appear to be exceptionally large nor exceptionally small for a hydrologic record of this length. Implications for effective discharge of bedload transport

In the Wolman & Miller (1960) approach to magnitude-frequency analysis the relative importance of various discharges is assessed by determining the fraction of the total 60

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3·0

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7·0 9·0 11·0 13·0 Discharge (m'/s)

15·0

17·0

19·0

Figure 3. Frequency distribution of flows at the SouthMountain site.

HYDROLOGIC CHARACTERISTICS OF A SMALL DESERT MOUNTAIN STREAM

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Figure 4. Log-Pearson Type III flood frequency curvefor the South Mountain site. Dots are peak annualfloods (plottedusingCunnane's (1978) formula), pluses are values predictedusingRoeske's (1978) equations. sediment load transported by flows of various frequencies. Ideally, one should use longterm measurements of sediment transport in this type of analysis. However, this information is rarely available due to the lack of field data on bedload transport. An alternative approach is to compute bedload transport rates using appropriate transport functions (Pickup & Warner, 1976; Andrews, 1980). This procedure is used here to explore the effect of the non-lognormal flow distribution on the magnitude and frequency of bedload transport near the South Mountain site over the last 24 years. Due to the lack of field data this type of analysis is partly theoretical in nature. However, it is similar to paleohydraulic studies that rely on empirical functions to estimate flow characteristics. Results of the analysis are comparable with other investigations that have used similar procedures (Pickup & Warner, 1976; Andrews, 1980). Shick and Lekach (1983) demonstrated that Bagnold's model (1980) provides reasonable first-order approximations of transport rates (ib ) for flood waves in desert mountain channels. This model assumes that an unlimited supply of sediment is available and that significant armoring of the bed does not exist. The deep, poorly sorted alluvium on the bed of the South Mountain channel upstream from the gaging site generally satisfies these requirements. The function has the form: i b a (w - we»)'5d-o'67D-o'5 where w is the stream power per unit bed area for the discharge at the midpoint of each flowclass, We is the threshold value at which bedload transport begins, d is the mean depth of flow, and D is the modal size ofthe bed material. In this study, it was assumed that mean depth is proportional to water stage. Also, D so of the unimodal bed material distribution was substituted for D, as recommended by Bagnold (1980). Computation of We and the corresponding critical discharge are outlined in Table 2. Despite the non-lognormal shape of the probability density function of stream flow, the histogram for the magnitude and frequency of computed bedload transport exhibits a form that is approximately lognormal (Fig. 5). The peak of the curve corresponds to the 1·5 to 2·0 m 3/s discharge class. The effective discharge lies somewhere within this interval. If the total period of record is considered (including periods of no flow), the lower limit of the effective discharge class is equalled or exceeded 0'011% of the time (0'04 days/57 min per

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year), whereas the upper limit is equalled or exceeded 0·008% of the time (0·03 days/ 43 min per year). These figures are an order of magnitude smaller than most values reported for humid temperate rivers (e.g. Pickup & Warner, 1976; Andrews, 1980). A somewhat different picture emerges if the recurrence interval of the effective discharge is considered. The recurrence intervals for discharges at the upper and lower limits of the effective flowclass are 2·38 and 2·53 years, respectively (Fig. 4). These figures closely approximate the values reported for perennial streams (e.g. Pickup & Warner, 1976; Andrews, 1980). They indicate that on an annual basis the number of effective flows for this desert mountain stream does not differ greatly from that for humid-region rivers. Over a given time span, both types of streams will on average experience the same number of effective events, but the total duration of the effective flows is much less for the desert mountain channels. The infrequency of effective flow probably delays recovery times and one might question whether hydrologic processes and channel form achieve equilibrium over short periods. To evaluate this issue for the South Mountain site, the bankfull discharge (Qbk) of the active channel was estimated by two methods. The first approach is based on the equation of continuity: Qbk = AV

where A is the cross-sectional area of the active channel (1·14 nr') and V is the mean velocity of bankfull flowcomputed by Bathurst's (1985)resistance formula (Table 2). This method produced an estimate of 1·99 m3/s for Qbk. A second independent estimate of bankfull flow was computed using Williams (1978) empirical formula: Qbk = 4·0 A l'll SO'l8 where S is channel slope, which yielded Qb = 1·68 m 3/s. Both estimates fall within the discharge interval containing the computed effective discharge. This test is only approximate, but it suggests that the capacity of the active channel is adjusted to the range of discharges that transported the largest fraction of the total bedload over the last 24 years. Discussion The analysis above indicates that events of moderate magnitude may mold the active channel in this desert mountain stream; however, it does not disprove the geomorphic effectiveness of large floods. In fact, other factors appear to affect the gross morphology of the South Mountain channel because the streambed near the gaging site is incised 3 to 4 rn below the surface of an alluvial fill (Fig. 6). Although the exact timing of this incision is unknown, it certainly predates the period of hydrologic record (Pewe & Shank, 1973).

HYDROLOGIC CHARACTERISTICS OF A SMALL DESERT MOUNTAIN STREAM

Table 2. Calculation of critical stream power per unit boundary area and critical discharge for incipient motion ofbedmaterial at theSouth Mountain gaging site Step l.

Determine the critical shear stress (T<) for incipient motion using Shield's (1936) function with () = 0'044 (Dingman, 1984, pp. 155156). Te = ()g(ps - Pw)Dso = (0'044)(9'81)(2650 - 1000)(0'0028) = 1·99N/m2 where () = dimensionless Shields parameter g = acceleration due to gravity (m/s2 ) Ps = density of sediment (ktm3) Pw = density of water (kg/m ) D so = median grain size of bed material (m)

Step 2. Determine velocity at incipient motion (a) Determine hydraulic radius of flow at incipient motion. At critical shear stress - T< = PwgRS where R = hydraulic radius of flow S = channel slope (0'025 m/m) Thus, R = 1'99 (1000)(9'81)(0'025) = 0·008 m (b) Determine shear velocity (V*) at incipient motion

V* =

v'T/Pw

=

J1000

l '99

= 0·045 m/s (c) Use Bathurst's (1985) equation for steep mountain stream to compute mean flow velocity, assuming that mean depth (d) equals hydraulic radius (R) in this broad channel

v

= V*(5'6210g(dlD 84)

+ 4)

= 0'045(5'62 10g(0·008/0'013) + 4)

= 0'13 m/s where D 84 = particle size coarser than 84% of bed material (m)

Step 3. Compute critical power per unit boundary area (we) for incipient motion We

= T
= 0·26 W/m 2

Step 4.

Compute discharge (Q) at incipient motion We = (PwgQS)1W where W = channel width (m)

Thus,

Q = (W w<)/(PwgS) (8'8)(0'26) (1000)(9'81)(0'025) = 0·01 m 3/s

159

160

B. L. RHOADS

Figure 6. Channel entrenchment immediately upstream from the gaging site.

Welsch (1977) critically evaluated several alternative explanations for entrenchment of mountain streams near Phoenix, including tectonic uplift, regional base level changes, human impacts, climatic change, and episodic erosion. He concluded that the incision is probably related either to an increase in flood frequency or to natural cycles of cutting and filling (episodic erosion) because (I) the mountains of the Phoenix Basin are tectonically inactive; (2) most of the mountain streams, including the one examined here, have local base levels and are not affected by aggradation and degradation on adjacent valley floors; and (3) human impacts on upland watersheds are minimal. Shick (1974) proposed that incision of desert mountain channels may be caused by highly-erosive superfloods. Envelope curves indicate that the maximum flood for a 4'5km 2 basin in the southwestern United States can be as great as 500 m 3/s (Crippen & Bue, 1977; Costa, 1987). The exact recurrence interval of such an event is unknown, but this value is over 10times larger than the magnitude of the IOO-year flood from the log-Pearson Type III analysis (44 m 3/s) and almost seven times greater than the SOO-year flood estimated with Roeske's (1978) regional flood-frequency equation (75 m 3/s). Such an event would have tremendous erosive potential, particularly if it exhausted or severely reduced the supply of available sediment in upper portions of the basin, producing nonequilibrium transport conditions downstream. The frequent occurrence of several smaller events could have the same effect. In other words, incision is likely to occur when a supplylimited situation develops in the basin due to a climatically-related increase in the frequency of large floods, the random occurrence of several high-magnitude, low frequency events over a short period of time, a single superflood, or complex response and associated episodic erosion/deposition. Between flood episodes channels in arid environments generally experience net aggradation (e.g. Emmett, 1974; Hereford, 1984; Schumm et ai., 1987). The nature of equilibrium adjustments in this incised desert mountain stream may depend on the specific aspect of channel form under consideration (Fig. 7). Following an erosional episode the active channel may adjust rather quickly (several years to a few decades) to the prevailing intererosional hydrologic regime. However, the healing of gross changes in channel morphology (i.e. entrenchment) via slow net aggradation probably requires much longer periods (decades to centuries). According to this scenario, the

HYDROLOGIC CHARACTERISTICS OF A SMALL DESERT MOUNTAIN STREAM

161

'Healing' time far gross (incised) channeI morphology ._--

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Figure 7. Diagramillustrating hypothetical behavior of a desert mountain stream. Following an erosional event that produceschannelincision, the morphology of the active channel(dashedline) adjusts to prevailing hydrologic conditions (A-B). Slow net aggradation during the intererosional interval initiateshealing of incised channelmorphology, but due to the longtime periodsinvolved (A-E), this process is probably completed only rarely. If erosional events occur frequently, the morphology ofthe active channelmayalsobe transient (C-D). morphology of the active channel commonly reflects prevailing hydrologic conditions, whereas the gross morphology of the channel is usually transient (Fig. 7). Thus, both

equilibrium and disequilibrium of river morphology may occur in a given fluvial system at the same time, with the active equilibrium channel superimposed on a palimpsest disequilibrium morphology. Channel forms of this type have been observed in other desert mountain streams in the Phoenix Basin (Rhoads, 1986b). This conceptual framework combines the Wolman & Miller (1960) and Wolman & Gerson (1978) concepts of geomorphic effectiveness by looking at the active and gross channel morphologies and their corresponding formative events separately. Conclusion This paper sheds light on the short-term magnitude and frequency of fluvial processes in desert mountain streams in southern Arizona and perhaps other similar settings in the southwestern United States. The 24-year hydrologic record for the South Mountain site clearly shows that flow of any magnitude is rare in desert mountain streams. The lack of base flow in these streams produces a flow frequency distribution that approximates a oneparameter gamma distribution. Computation of bedload transport rates for each discharge class indicates that the transport frequency distribution is lognormal, but that the shortterm effective event is equalled or exceeded much less frequently than it is in humid-region rivers. Although the South Mountain record is one of the longest available for a desert mountain stream, it is far too short to include the total range of geomorphically-significanr flows in arid environments. Thus, the analysis of this record perhaps raises more questions than it answers. In particular, the geomorphic effects of high-magnitude floods on desert mountain channels and subsequent 'healing' or adjustment during intererosional episodes must be reliably documented. This investigation suggests that adjustment between active channel form and the prevailing hydrologic regime is possible over the short-term, but that this equilibrium form is commonly inset into a palimpsest disequilibrium morphology produced by infrequent, catastrophic events. This model must be tested against long-term field observations of a desert mountain channel that has recently experienced an erosional event.

162

B. L. RHOADS

I thank Fran Jelinek, Water Resources Division, U.S. Geological Survey, Phoenix, AZ, for his assistance in obtaining the strip charts for the water stage recorder at the South Mountain gaging sites. Melissa Records drafted the figures.

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