314
Climatic Geomorphology
Figure 13.34. The headcut reach of an ephemeral stream eroded in a valley fill over a gypsum bedrock.
Barranco de Villafranca, Villafranca de Ebro, Zaragoza Province, Ebro Basin, Spain.
flows (Schumm et al., 1984). This downcutting may form continuous or discontinuous
gullies (Figure 13.37). Discontinuous gullies occur in relation to a slope threshold at which
the gully does not have sufficient transport capacity to continue forming (Schumm and
Hadley, 1957). In the southwestern USA, gullies with vertical walls are termed arroyos
(or ramblas in Spain). These morphological features have been the subject of numerous
studies in this region (Cooke and Reeves, 1976), with the aim of determining the causes of
~L==:E?=~, ,, --~ Small fans and
~
II "lo"e
-"
~EE~,
sheetflo'w reach
Braided
~,, .va ey ~ ~ ......
~,~~~
~Apexof
r, "~runk stream channel
Headcut "~
reach
~
Figure 13.35. Characteristic erosional and depositional reaches of a diagrammatic channel fan of a
discontinuous ephemeral stream (Bull, 1997).