Barriers Against Lava — MACDONALD 
261 
ume of approximately 600,000,000 cubic yards. 
This latter volume is more than twice that of 
the 1881 flow, and 6 times that of the flow of 
1942. The distance from the source of the 
1859 flow to the point where it entered the 
ocean is 32 miles. The vents of the 1942 and 
1881 flows are 28 and 30 miles, respectively, 
from the shore at Hilo. If either the 1881 or 
the 1942 flows had had a volume equal to that 
of the 1859 or 1950 flows, the lava almost 
certainly would have entered Hilo Bay, and 
doubtless would have overrun much of the 
city. 
In the vicinity of Hilo, lava flows of geolog- 
ically recent age rest on a bed of yellow ash 
(Stearns and Macdonald, 1946: 63-78), and 
early flows of this group buried charcoal that 
has been shown by radio-carbon dating to 
have been formed about 2,000 years ago 
(Macdonald and Eaton, in preparation). It is 
estimated that during the interval since then 
about 20 or 25 lava flows have entered the 
Hilo area. Thus, based on these crude statistics 
as well as on the historic record, an average of 
about one flow per century can be expected to 
enter the city of Hilo. Probably about one of 
every three such flows will enter the bay. The 
last flow to enter the present city was that of 
1881, and no flow has entered the bay since 
sometime previous to 1800. Obviously, these 
figures are inadequate for the determination 
of the mathematical probability of the en- 
trance of lava into the city or harbor within 
any given length of time; but within their 
limits they suggest that a flow may be ex- 
pected to enter the city within the next 25 
years, and to enter the harbor within the next 
century. No one can predict when this may 
happen — whether within the next 5 years or a 
century or two hence — but the threat is ap- 
parent and the implications to the economy of 
the island demand consideration of protective 
or palliative measures. 
The volume of water in Hilo harbor, and 
especially that in the deep ship channel, is 
comparatively small. The total volume east of 
a line connecting the end of the breakwater 
with the mouth of the Wailuku River is ap- 
proximately 45,000,000 cubic yards, and in 
the same area the central channel below a 
depth of 5 fathoms has a volume of only about 
3,000,000 cubic yards. Part of any flow enter- 
ing the harbor would project above sea level, 
of course, and part would occupy the shallow 
margin of the bay, but the topography of the 
bay floor would guide the advancing flow di- 
rectly into the most important part of the 
harbor — the ship channel. Once in this sub- 
merged valley the lava would tend to spread 
along it. Both the natural valley wall north of 
the channel, and the breakwater, would serve 
as barriers to confine the flow to the harbor. 
Thus 100,000,000 cubic yards of lava entering 
the harbor almost certainly would make it un- 
usable, and half that volume probably would 
have the same result. Indeed, a very much 
smaller volume entering the ship channel, as 
it very probably would do, would cause 
serious damage. 
The loss of Hilo harbor would be disas- 
trous to the present economy of much of the 
island of Hawaii, for there is no other harbor 
in that part of the island capable of handling 
the cargo that moves through the port of Hilo. 
Furthermore, the loss might well be perma- 
nent. The congealed lava in the bay could not 
to any large extent be removed by simple 
dredging, and a difficult and very costly 
blasting operation would be necessary to clear 
the harbor. 
It should be noted that in time of eruption 
the supply of fresh water for Hilo may present 
a serious problem. Most of the city’s water 
now comes from the Wailuku River. A lava 
flow entering the Wailuku drainage basin 
might greatly reduce the volume of available 
water and render the remainder unusable with- 
out special treatment. During the 1855 erup- 
tion the river water became much discolored 
by organic matter from burned vegetation, 
but at that time it caused no trouble because 
the city’s water was obtained from springs. 
The possibility of lava flows seriously damag- 
ing Hilo’s water supply was pointed out sev- 
