Several small rivers flow into the strait along this section of coast. Small, shallow draft coastal vessels use these small rivers.
Between Ujung Tamiang and Tanjung Tanjung, about 96 miles SE, and then to Tanjung Sinaboi, about 115 miles farther SE, the low swampy coast is intersected by numerous small rivers, few of which are navigable. High mountain ridges rise in the interior and are clearly visible.
The depth curves generally follow the contour of the coast with the 10m curve lying about 1 to 5 miles offshore, except in the bays and inlets.
The SE part of the Strait of Malacca constricts to a width of 37 miles between Pulau Sinaboi and Tanjong Ru, on the Malaysian coast. The fairway is fouled by a series of narrow detached banks with depths of 11m and less.
Although the Strait of Malacca is within the limits of the Northeast Monsoon and Southwest Monsoon of the Indian Ocean, the winds are variable because of the high land on both sides. Land and sea breezes are regular on both coasts. In the offing, the monsoons are only regular when they are at their height in the adjacent sea area. However, the wind is moderate in the strait and only lasts for part of the day.
The monsoons become more regular near Singapore.
Between Acheh Head and Ko Phuket, the Southwest Monsoon commences in the latter part of April or the early part of May, and ceases in October. Calms and variable winds frequently prevail in November.
The Southwest Monsoon seldom blows far into the strait.
During this season, variable winds, chiefly from the SE and SW, prevail in the middle of the strait, with periods of long calms.
On the Sumatera side, light winds and calms prevail, and heavy squalls from the land are experienced during the night.
Fewer calms are experienced on the Malayan side and there are seldom any squalls. Variable land and sea breezes are usually experienced.
During the Southwest Monsoon, the weather is generally cloudy and stormy especially when the monsoon is at its peak.
Sumatras, or squalls from the SW, are more common during the Southwest Monsoon than during the Northeast Monsoon.
They generally occur during the first part of the night and are accompanied by sudden severe winds, with thunder and lightning.
They are more frequent on the N coast of Sumatera and along the Malaysian coast between Parcelar Hill and the Karimum Islands. Here they usually blow for 6 to 8 hours at a time as a strong, or moderate gale. Their characteristic is that of an arch squall.
Northwesters are not as frequent as the Sumatras. They are most common during the Southwest Monsoon and occur in the NW part of the strait but sometimes are felt as far SE as Singapore Strait. Severe high winds blow at the beginning of the storm but their strength soon abates. They are generally preceded by a black cloud arch, which rises rapidly from the horizon toward the zenith and are usually accompanied by thunder, lightning, and heavy rain.
The Northeast Monsoon prevails in the W entrance of the Strait of Malacca from November to April, which is considered the fair season. The weather is more settled at this time. There are seldom severe squalls and there is less thunder, lightning, and much less rain than in the other season.
In November, the winds are variable, frequently from the NW and W, although occasionally the NE winds set in November.
From this period to March, the Northeast Monsoon is the strongest, but at times NW and W winds of 1 or 2 days duration have been experienced in every month when the Northeast Monsoon should prevail.
Late in March, the NE and N winds become light and variable, with strong land breezes at night. On the Malaysian side these breezes commence between 2000 and 2200 and last for 4 or 5 hours, sometimes blowing all night.
This is generally the case between Mount Formosa and Cape Rachado. Calm winds are less likely to exist on the Malaysian side than on the Sumatera side of the strait.
The Strait of Malacca is relatively shallow, with the greater part of the area having depths of less than 73m. The main movement of water is from tidal influences.
Throughout the year, there is a residual predominantly NW current in the strait.
During the NW monsoon, part of the S current in the South China Sea rounds the S extremity of the Malay Peninsula and sets NW through the Strait of Malacca. During the period of the Southwest Monsoon, part of the current which flows through Karimata Strait and into the South China Sea, branches off to the NW into the Strait of Malacca. This NW current is also present during the transition months of April and October although at these times it becomes weaker and less constant.
As the NW monsoon becomes well established there is some evidence in some of the winter months for an counterclockwise circulation in the N parts of the strait, N of about 3°N. This circulation weakens during the April transition.
When the Southwest Monsoon becomes established, a clockwise circulation probably results over the same area during the period June to October, with a maximum effect in August.
Though the predominant direction in the strait is NW, currents from all directions have been reported and the percentage frequency of the predominant flow is never high.
The current is most constant during the period January to April and is least constant from May to August. A number of observations, report rates of less than 1 knot.
Some have been reported more than 1 knot and no currents have been reported in excess of 2 knots.
The tides on the coast of Sumatera covered by this sector are chiefly semi-diurnal in character. However, on the N and NE coasts the diurnal tidal system of the South China Sea is felt at times, and when the highs and lows of both systems coincident springs, greater highs and lows are experienced.
The flood tidal current sets E on the N coast of Sumatera; the ebb tidal current sets W. At springs the current rarely exceeds 2 knots; at neaps they are sometimes imperceptible, except at the points or over banks and narrow channels.
The currents are also affected by the constant current out of the Strait of Malacca, which takes a W direction along the N coast, through Malacca Passage, and out through Bengal Passage, so that for the greater part of the year the ebb current is longer and stronger than the flood current.
As a result of the prevailing wind, when the water is rising or falling during the NW monsoon, there may be no E set for a day or more; conversely, the flood or E current runs longer and stronger during the Southwest Monsoon.