861 
of the andesite hills possibly accompanied by local 
upwarping. A blocking of the main drainageway, the 
K.Progo, has been the result. 
This is borne out by a survey of the long profile 
of the K.Progo by Schmidt (1934), reproduced in 
fig. 3 , showing a distinct bulge upstream of the 
debouchure of K.Krasak, by as much as 50 metres. 
Van Bemmelen had little doubt that this phenomenon 
has been accountable for the origin of a ’Borobudur 
Lake’ , ponding the water upstream of the blockade. 
The lake may have had various expansions and 
retractions as a result of posterior changes in the 
ponded runoff. At times, its extent may have been 
considerable. From the present morphology it can be 
judged that these expansions and retractions have 
mainly caused shifts in the north bank of the lake. 
The area south of the present Borobudur temple has 
been covered for the longest period- as is also 
borne out by the thickness of the lake deposits. 
5. PRESENT DRAINAGE CHARACTERISTICS 
5.1 K.Sileng 
The plain stretching south of the Borobudur is 
drained by the K.Sileng, which flows out of the 
Menoreh Hills on a northerly course. It soon bends 
eastward, receiving tributaries only from the 
south, from the Hills. This plain is marked ’LP’ 
(for Lake Plain) in fig. 4 and 5; its level is 240 
metres a.s.l in its eastern and central parts. 
East of the centre, the K.Sileng sinks into the 
lake plain in a course of incised meanders, while 
assuming a southeasterly direction that brings it 
back to the foot of the andesite hills. 
Finally it breaks through the expanse of the plain 
in a gorge of some 30 metres depth and debouches 
into the Progo river. 
lower reaches of the K.Pabelan, one of the lahar 
rivers from the Merapi. However, its very nature of 
lahar river renders a correlation of its terraces 
with those of the Progo river, somewhat hazardous. 
The Pabelan debouches with a steepened gradient 
into the Progo (figs. 1 and 4). Other Merapi- 
derived rivers show the same phenomenon of steep, 
even hanging, debouchure. The K. Batang shows a 
downward jump of over 10 metres at its debouchure 
into the Progo. 
This should not be ascribed to excessive apport of 
material from the Merapi, which would cause block 
ing and ponding rather than a hanging debouchure, 
but rather to uplift on a local scale in this 
sector of the Progo river. 
5.4 Kali Tangsi 
The Tangsi river, entering the area of study from 
the northwest (figs 1 and 6), displays a curious 
phenomenon. About 3 km northwest of the Borobudur, 
it takes a 90-degree bend to the northeast, and 
continues on this course until it debouches into 
the Progo river. Its channel in this tract is some 
15-20 metres below the level of the plain ( here at 
270 m. a.s.l and designated ’HP* (for Higher Plain) 
in figs 4 and 5). 
On the way, it receives the K. Merawu, which also 
comes from the northwest and debouches at a 90- 
degree angle into the K. Tangsi. 
This trend taken by the K. Tangsi is also visible 
on the opposite bank of the Progo river, where an 
affluent from the northeast takes roughly the same 
trend, only in the opposite sense. 
As can be seen in figs. 1 and 5 , this phenomenon 
leads to the concentration of runoff from the 
northwest and north, at one single point into the 
Progo river. 
5.5. Terraces of the Sileng and Progo rivers 
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ing north and 
5.2 The LD drainageways 
Affluents from the north into the K.Sileng are 
absent ( with one exception still deriving from the 
northern andesite hills). Instead, some wide, 
flatfloored and very shallow channel patterns are 
discernable, barely below the LP level. These are 
designated LD (for Lake Drainage) channels in fig. 
4, and are considered to represent a system of 
drainageways from a late phase of the dwindling 
lake. 
If any well-defined main channels have ever existed 
in these vales at all, they are not distinguisable 
anymore because all the water is presently diverted 
into the irrigation systems of this intensive rice 
cultivation area. 
5.3 The K.Progo 
The Kali Progo displays incised meanders to a depth 
of about 30 metres below the LP level; along its 
course through the area, at least two terrace 
levels can be distinguished, marking corresponding 
phases of rest in the incision. 
The incision of the meanders of the Sileng and 
Progo rivers points to an uplift of the area after 
the emptying of the lake, or perhaps starting in a 
late phase of its existence. 
Corresponding terrace levels are observed in the 
Clearly discernable terraces are present along both 
the Sileng and the Progo rivers. They are separable 
into two main levels, possibly a third one can be 
distinguished. 
These two or three main levels point to as many 
phases of rest in the incision of the rivers. This 
incision is, as will be discussed later, ascribed 
to uplift on a regional or local scale. The Kali 
Sileng breaks through the LP level in an antecedent 
gorge which shows that the river had its present 
course with respect to the lake plain, prior to the 
uplift-cum-incision. 
A notable feature is also, that the LD drainageways 
drain into the Kali Sileng and not into the Kali 
Progo. 
6. STRUCTURAL IMPLICATIONS 
The lineaments along the K.Tangsi point to a struc 
tural (=fault(line)) control of the tract of the 
river that runs to the northeast. Its counter part 
from the northeast, on the other bank of the Progo, 
is likewise structurally controlled. 
The relative configuration of the HP and LP plains 
points to a relative uplift on the southeast side 
of the Tangsi lineament. This is corroborated by 
the straight and gorge-like incision of the K. 
Progo downstream of the Tangsi debouchure, in 
contrast to its meandering course in a wider 
(though also incised) valley upstream of this 
point.