Arizona Geological Survey OPEN-FILE REPORT OFR 08-06

Arizona Geological Survey
deposits located above the active channel are commonly buried by fine sediments either from initial
deposition and subsequent abandonment, soil accumulation, or overbank deposition. Coarse cobbleand
boulder‐dominated deposits derived from reworked debris‐flow deposits (unit C) are found in
channel bottoms or forming channel banks. These deposits generally do not exhibit sorting when viewed
in channel banks, but also do not exhibit defining debris‐flow deposit characteristics. Unit C deposits
have probably been winnowed and reworked by fluvial processes.
Soldier Canyon
Soldier Canyon (Figure 1, map DM‐DF‐1B) is drained by Soldier Wash, which flows under the Catalina
Highway through two 6 ft (2 m) box culverts and continues downstream to exit the mountain front near
the Mount Lemmon Short Road, approximately ½ mile (1 km) below the box culverts. Pleistocene
debris‐flow deposits (unit I) were observed along Soldier Wash above and below the Mount Lemmon
Highway at the box culverts. In Soldier Canyon, these are the highest standing debris‐flow deposits,
forming terraces up to 16 ft (5 m) above the active channel. Older Holocene to latest Pleistocene debrisflow
deposits (unit YI) are inset 3‐7 ft (1‐2 m) below I deposits. On the Soldier Canyon fan, downstream
of the Mount Lemmon Short Road, YI deposits are farthest removed from the active channel area.
Intermediate debris‐flow deposits (unit Y1) are generally found 3 to 10 ft (1‐3 m) above the active
channel near the mountain front. On the Soldier fan they define the lateral extent of recent channel
migration. Young debris‐flow deposits (unit Y2) are generally found less than 3‐5 ft (1‐1.5 m) above the
active channel and are inset below Y1 deposits. Very young debris‐flow deposits (unit Y3) are found
within or adjacent to the active channel upstream of the fan apex. Coarse boulder bars of uncertain
origin (unit B) were mapped farther out on the fan, near Snyder Road. These deposits probably
represent reworked debris‐flow deposits. Unit B deposits were used to define the general downstream
extent of debris flows on the Soldier fan.
During mapping we observed an open septic‐system test pit, approximately 6 ft (2 m) deep, on a late
Pleistocene to early Holocene alluvial fan surface. The fan surface was smooth and fine‐grained with no
observable evidence of debris‐flow deposits. The upper 2 ft (0.5 m) of the pit was composed of sand, silt
and fine gravel, presumably deposited by sheetflooding on the fan. The bottom 5 ft (1.5 m) was entirely
composed of unsorted boulders likely derived from debris flows (Figure 15). This exposure demonstrates
that subsequent burial has obscured some older debris‐flow deposits. While YI debris‐flow deposits are
not mapped very far south of the Mount Lemmon Short Road, it is likely they are present below the
surface.
Soldier Canyon was heavily impacted by the July 2006 storms. Webb and others (2008) documented 56
hillslope failures within the Soldier Canyon watershed. These hillslope failure coalesced into debris flows
that travelled down canyon, under and over the Catalina Highway, and down Soldier Wash. Comparisons
of orthophotographs taken in 2002 and 2007 show significant channel widening and alluvial deposition
resulting from the 2006 debris flows and floods (Figure 16). Sediment from at least two debris‐flow
pulses were deposited on the Soldier fan (Figure 17).
24