Historical flooding in Lytle Creek between the apex of the old fan at the front of the San Gabriel Range and the Santa Ana River to the south has been confined to the incised channel (also see Appendix A). The channel has cut into unconsolidated alluvial deposits from the canyon mouth near the apex of the old fan to the El Cajon Wash and even further downstream to near the confluence with the Santa Ana River. Some lateral movement of the steep cut banks is suggested by reports of bridge failures during past flooding at San Bernardino (McGlashan and Ebert, 1918) and reports of channel movement just above the canyon mouth (Troxell, 1942). The largest known discharge from the 119.9 km² (46.3 mi²) drainage basin above the U.S. Geological Survey stream gaging station near Fontana, California (Number 11062000), was 1,017 m³ (35,900 feet³) per second on January 25, 1969 (Chin et al., 1991). The capacity of the channel about 2.4 km (1.5 mi) below the gaging station and just below the mountain front is about 3 times the magnitude of the 1969 flood.

The present incised channel (1995) of Lytle Creek is very similar to the channel reported by Eckis (1928). DMA Consulting Engineers (1985) also reported that the flow path of major floods in upper Lytle Creek was unchanged from 1935 to 1969. A comparison of flow paths shown on aerial photographs of October 12, 1967 (DMA Consulting Engineers, 1985), and an aerial photograph of August 29, 1989, indicates no movement of the channel banks. Thus, using channel conditions suggested in the account of flooding by Troxell (1942), the channel of Lytle Creek has been deeply incised since at least 1862, and the path of flow has not changed.

The soil has a well-developed, grayish-brown surface layer of stony or gravelly loamy sand. The underlying material is brown, very stony sand to a depth of 1.5 m (5 feet). There is no active surface on the Lytle Creek alluvial fan.

The Lytle Creek alluvial fan is not subject to alluvial fan flooding processes as defined by the committee. The incised channel conveys flows at shallow depths and high-velocity during floods. Flood control structures installed over the past 40 years have prevented large-scale channel migration. Although substantial erosion and deposition occur in the main channel, flow path uncertainty is minimal.

The floodplain is topographically bounded on both sides. This confinement is mostly because of channel incision and to a lesser degree because of the spur dikes and levees a few hundred feet above and below Interstate Highway 15. Without the presence of the constructed spur dikes and levees, flood mitigation measures such as setbacks and elevation on fill might not reliably mitigate the hazard.

Lytle Creek is entrenched in a classically shaped relict alluvial fan. It does not, however, fit the committee's definition of alluvial fan flooding. Sediment delivered by watershed erosion processes is conveyed through the fan, but large sedimentation events are tentatively considered

Front Matter (R1-R10)

Summary (1-5)

Introduction (6-28)

Flooding Processes and Environments on Alluvial Fans (29-50)

Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding (51-82)

Applying the Indicators to Example Fans (83-132)

Conclusions and Recommendations (133-138)

A Characteristics and Hazards Reported in Published and Unpublished Accounts of Alluvial Fan Flooding (139-152)

B Sources of Data (153-163)

C Biographical Sketches of Committee Members (164-166)

D Glossary and List of Acronyms (167-172)