Figure 1.  Map showing regional geologic framework and 

location of Forest Falls.  Faults modified from Matti and  

others (1992) and Matti and Morton (1993).  Faults shown in 

colors are strands of the San Andreas Fault; red indicates 

modern traces of the San Andreas Fault.  BPF, Beaumont Plain 

fault zone; CHH, Crafton Hills horst-and-graben complex; GHF, 

Glen Helen Fault; ICF, Icehouse Canyon Fault; MVF, Morongo 

Valley Fault; SCF, San Antonio Canyon Fault; VT, Vincent 

Thrust.



Figure 2.  View looking south across Mill Creek and the south 

face of Yucaipa Ridge.  Rock Creek is the relatively small 

drainage on the left side of the photograph that does not 

reach the summit of Yucaipa Ridge.  To the right of Rock 

Creek is Snow Creek canyon with a broad headward drainage 

area.  The relatively narrow elongate drainage to the right 

of Snow Creek is Rattlesnake Canyon.  Note the bare branching 

channels that extend nearly to the summit of these three 

drainages.  Photograph by I.K. Curtis.



Figure 3.  Simplified geologic map of the Forest Falls area 

showing generalized distribution of the debris flows and 

debris avalanche of July 11 and 13, 1999.



Figure 4.  Broad debris flow levee on the lower part of Snow 

Creek fan where the Valley of Falls Road crosses Snow Creek.  

View is to the west.  Here the levee is several hundred feet 

in width.  Photograph was taken at the crest on the east side 

of the levee; the near car is at the center of the channel 

and the far car is on the crest of the west levee.  These 

broad debris flow levees give rise to the rather anomalous 

setting of a channel occupying the crest of the topographic 

high.



Figure 5.  Medial part of Snow Creek fan.  In the lower right 

foreground is Snow Creek debris flow channel with a newly 

formed, sharp-crested debris ridge.  This stretch of the 

channel is where the debris avalanche of July 11, 1999, shot 

over the channel wall degrading the debris flow levee and 

removing vegetation.  The sharp crested ridge along the 

channel was formed by debris flows on July 13, 1999.  The 

house in the background was struck on the left by the 1st 

debris avalanche surge and struck on the right by the 2nd 

and/or 3rd surge.  Note the flattened trees oriented parallel 

to the 2nd(?) and 3rd debris avalanche waves.  Photograph 

taken July 15, 1999.



Figure 6.  Sharp-crested debris flow ridge on the medial part 

of Rattlesnake Creek fan.  The age of this ridge is over 50 

years based on the size of trees growing on the crest of the 

ridge.



Figure 7.  Rocky snout of a slow-moving debris flow on the 

proximal part of Snow Creek fan.  This debris flow is over 

250 years based on the age of trees growing on the deposit.



Figure 8.  Abandoned debris flow channel on the medial part 

of Rattlesnake Canyon debris flow fan.  This channel was 

abandoned relatively recently based on the age of the cedar 

trees growing in the channel.



Figure 9.  Steep front of broad debris flow snout on the 

medial part of Rattlesnake debris flow fan.  Cedar trees at 

base of the flow predate the flow and the smaller cedar trees 

on the upper surface of the flow postdate the flow.



Figure 10.  Snout of narrow debris flow on medial part of 

Rattlesnake debris flow fan. Cedar trees at base of the flow 

predate the flow and the smaller cedar trees on the upper 

surface of the flow postdate the flow.



Figure 11.  House damaged by a relatively fast moving debris 

flow of 1997.  The 'clean' nature of the deposit suggests a 

relatively high water content to the flow.  Photograph by 

W.E. Reeder.



Figure 12.  Vehicles destroyed by relatively fast moving 

debris flows of 1997.  The 'clean' nature of the deposit 

suggests a relatively high water content to the flow.  

Photograph by W.E. Reeder.



Figure 13.  Utility lines downed by the debris flows of 1997.  

Photograph by W.E. Reeder.



Figure 14.  House partly inundated by slow moving debris flow 

of 1997.  Note there appears to be no structural damage to 

the house produced by the slow moving debris flow.  

Photograph by W.E. Reeder.



Figure 15.  Snout of a slow-moving debris flow of July 11, 

1999, Rattlesnake fan.  Looking south up the fan.  The larger 

boulders are about five feet in length.  Some watery matrix 

is oozing from the left side of the flow.  Photograph taken 

July 12, 1999.



Figure 16.  Remnant of the distal part of the 1999, debris 

avalanche.  Note the 'clean' nature of the deposit.  The 

relatively high water content of the avalanche results in the 

'clean' aspect to the deposit.   Compare this deposit with 

the muddy deposits of the slow moving debris flow in Figure 

15).



Figure 17.  Rills on the upper slopes of Yucaipa Ridge.  

Intense rainfall of July 11 and 13, 1999 produced these 

rills.  The rills are about one foot in width.  Photograph 

taken July 15, 1999.



Figure 18.  Rills at the head of Snow Creek funneling down 

slope into a single channel.  Note the relatively small grain 

size to the hillside material and the extensive erosion of 

this material by the rilling.  This photograph is looking 

down slope from the site of Figure 17.   Photograph taken 

July 15, 1999, W.E. Reeder.



Figure 19.  Debris flow deposits in the upper part of Snow 

Creek at the proximal part of the debris flow fan.  There is 

no debris flow channel developed on the upper proximal part 

of the fan.



Figure 20.  Debris flow deposits on the lower proximal part 

of Snow Creek debris flow fan.  Here, on the right side of 

the photograph, is a new channel cut by the debris avalanche 

and debris flows of July 11 and 13.  In this reach the 

relatively fine-grained rilled debris incorporated boulders 

at the channel was cut. Compare this channel with the well-

developed channel in Figure 26. Photograph taken July 15, 

1999.



Figure 21.  Boulder transported by the July 11, 1999, debris 

avalanche.  The boulder is 16 feet in length and weighs 

approximately 70 tons.  The rocky debris to the sides of the 

boulder is the result of deepening Snow Creek channel after 

the channel was partly filled by the debris flows of July 13, 

1999.



Figure 22.  Snow Creek debris flow levee.  Debris flow 

channel to the left of the photograph.  The south side of the 

debris avalanche of July 11, 1999, crossed the levee where 

the shadows and small trees start.  In the foreground the 

avalanche removed vegetation and surface material.



Figure 23.  Excavation through the July 11th Rattlesnake 

Creek debris flows, Valley of the Falls Road.  Larger 

boulders are five-seven feet in length.  Photograph taken 

July 12, 1999, W.E. Reeder.



Figure 24.  Debris deposited across Valley of the Falls Road 

by the July 11, 1999, debris flows on Rattlesnake Canyon fan.  

These deposits cover the road for several hundred feet.   

Equipment (yellow) is in the process of removing the debris 

from the road.  Photograph taken July 12, 1999.



Figure 25.  Debris flow deposits, across Valley of the Falls 

Road.  Photograph taken July 12, 1999.



Figure 26.  Well-developed debris-flow channel, looking south 

up the medial part of the Rattlesnake Creek fan.  Debris 

flows of July 11and 13, 1999, added new debris to the crest 

of the levee on the right side of the channel.



Figure 27.  Looking up the blocked part of the Rattlesnake 

Creek debris flow channel.  Person is standing on the top of 

the snout of the flow that blocked the channel.  Figure 26 

photograph was taken from the position of the person.  Large 

boulder in center of the photograph is over 10 feet in 

length.  Note the tree trunks in the lower right corner of 

the photograph that were flattened and debarked by debris 

flows of July 11, 1999.



Figure 28.  Looking down Rattlesnake Creek channel that was 

abandoned on July 11 or July 13, 1999, from the position of 

Figure 27 photograph.  Light gray fluvial sand and gravel in 

channel bottom is the result of water draining from the 

debris flow that blocked the channel.  Large boulder behind 

the person is about 12 feet in length.



Figure 29.  Clasts of the July 11, 1999, debris flows perched 

in tree, Rattlesnake Creek debris flow channel.  The height 

of these clasts indicates the crest of the debris flows was 

over nine feet above the channel floor.



Figure 30.  Snout of a stabilized debris flow of July 11, 

1999, on the lower part of Rattlesnake fan.  This flow 

stopped on the surface of a larger flow that deposited debris 

earlier in the day.  Photograph taken July 12, 1999.



Figure 31.  Snout of a stabilized debris flow of July 11, 

1999, on the lower part of Rattlesnake fan. The gently 

sloping snout of this flow indicates the flow was relatively 

fluid.  Note the concrete-like nature of the debris flow 

matrix.



Figure 32.  Left side of house was removed by 1st and/or 2nd 

debris avalanche waves and the right side by the 2nd and/or 

3rd avalanche waves, July 11, 1999.  Note the 'swept' 

appearance in foreground produced by avalanche degradation 

where it removed both rocks and vegetation.  Large boulder on 

right side of the photograph is shown in Figure 21.  

Photograph taken July 12, 1999.



Figure 33.  Tree debarked by debris flows, Rattlesnake Creek.  

Scar above hand was produced by 1997 debris flows; scar below 

the hand was produced by a combination of both the 1997, and 

1999, debris flows.



Figure 34.  Upslope side of tree fragmented by the July 11, 

1999, avalanche. Note the rock embedded in the fragmented 

wood below the lens cap.



 Figure 35.  Upslope side of tree fragmented by the July 11, 

1999, avalanche.  Note the rock embedded in the fragmented 

wood below the right side of the pencil.



Figure 36.  Rock embedded and fragmented by impact of the 

high velocity debris avalanche of July 11, 1999.



Figure 37.  Wood splinter propelled by avalanche that was 

embedded in a tree about 300 feet below point where avalanche 

shot-over the channel.  Height of the debris avalanche at 

this location, marked by the debarked side of the tree, was 

about 1.5 feet below the wood splinter.



Figure 38.  Close up of the wood splinter.  Splinter is about 

nine inches in length.



Figure 39.  Tree scared by marginal part of the debris 

avalanche of July 11, 1999.  This tree is located about 50 

feet from the edge of the debris flow channel wall.  Scar is 

eight feet above ground surface and mud was splattered ten 

feet above the scar.



Figure 40.  Tree scared on the high side of the superelevated 

avalanche.  This tree is located on the outside of an open 

curve in the channel.  The arrow points to the position where 

the avalanche scared the tree.  Mud splattered to about eight 

feet above the scar.  Note the avalanche did not extend to 

the right of the tree.



Figure 41.  Arrow on the right side of the photograph is on 

tree is at the same position of the arrow on Figure 40.  The 

arrow on the left side of the channel (below end of brown 

building) is the maximum elevation of the low side of the 

superelevated avalanche.  The minimum elevation difference 

between the high and low side of the avalanche is 11 feet.  

This photograph was taken after the debris flows of July 13, 

1999, and the arrow on the left side of the photograph 

probably is the height of the debris flows of July 13, 1999, 

rather than the debris avalanche of July 11.  Thus the 

velocity determined for the debris avalanche at this site is 

a minimum velocity.



Figure 42.  Curve in Snow Creek debris flow channel where 

avalanche shot over the channel wall.  Note the 'swept' 

appearance of the degraded channel wall. Brown building in 

center of the photograph is that in the left side of Figure 

41.  Relative open curve in the channel to the left of the 

building is where the avalanche had a superelevation of a 

minimum of 11 feet. Photograph taken on July 12, 1999.



Figure 43.  Surface of Snow Creek fan degraded by the debris 

avalanche where it shot over the channel wall.  The 

rectangular area (arrows) is the foundation of the house 

completely destroyed and removed by the avalanche.  The 1st 

avalanche wave struck the end of the house at 90 degrees.  

The direction of the 1st avalanche 'wave' is indicated by the 

flattened tree that is parallel to and just beyond the far 

side of the foundation.  The fragments of the house are 

located to the right of the photograph parallel to the 

direction of length of the foundation and the flattened tree.  

The direction of the 2nd and/or 3rd avalanche 'waves' is 

indicated by the orientation of the flattened tree in the 

foreground.  The new sharp-topped debris flow ridge was 

produced by 'overflow' from the debris flows of July 13 is 

seen in the center of the bottom of the photograph.  

Photograph taken on July 15, 1999.



Figure 44.  Battered foundation of the house that was 

destroyed and removed by the debris avalanche.



Figure 45.  Close up of the foundation of the house.  Note 

the fragmentation of the foundation and the reinforcing rods 

bent into the direction of flow of the debris avalanche.



Figure 46.  Swept landscape produced by the avalanche.  Large 

boulder (arrow) moved in the avalanche is 16 feet in length 

(see Figure 21).  Note the 'swept' appearance of the surface.  

House in background was struck on the left side by the 1st 

'wave' and the right side by the 2nd and/or 3rd 'wave'.  

Photograph taken July 12, 1999.



Figure 47.  Houses destroyed by the first debris avalanche 

wave on the west side of the avalanche. Photograph taken July 

12, 1999.



Figure 48.  Destroyed houses down Snow Creek fan along the 

central path of the first debris avalanche wave.  Here the 

debris avalanche, no longer confined by a channel had spread 

out over a width of more than 100 feet. Photograph taken July 

12, 1999.



Figure 49.  Destruction of structures in the path of the 1st 

and perhaps 2nd avalanche 'waves' as the debris descended 

down Snow Creek fan.  This location down fan from the 

location of Figure 47.  Note the bent and stripped small pine 

trees oriented parallel to the flow direction of the 

avalanche.  Although the avalanche was slowing it still 

retained considerable energy. Photograph taken July 12, 1999.



Figure 50.  Two cars flattened by the debris avalanche are 

partly exposed in the mixture of rock and house debris.  Note 

the scar on the large pine tree produced by debris 'thrown' 

by the debris avalanche (see Figure 55 for detailed 

photograph of the scar).  Photograph taken July 12, 1999.



Figure 51.  Car exhumed from the site of Figure 50.



Figure 52.  Damage to structures produced down fan on the 

east side of the 3rd(?) wave of the debris avalanche.



Figure 53.  Damage to structures produced down fan on the 

east side of the 3rd(?) wave of the debris avalanche.  Note 

the fragmented foundation in the foreground.



Figure 54.  Damage to structures produced down fan at the 

east edge of the 3rd(?) wave of the debris avalanche.  Note 

the scaring on the left side of tree in the foreground.



Figure 55.  Scar produced by a thrown rock eight feet above 

the surface of the avalanche.



Figure 56.  Damage where the 3rd(?) wave of the debris 

avalanche spread-out down fan and was loosing velocity.  Note 

the relatively clean nature of the deposit.



Figure 57.  Fluvial deposits in the main channel of Mill 

Creek.  Note the rounded nature of the clasts in contrast to 

the angular clasts in the debris flow and debris avalanche 

deposits.