For every separate section of sewer, be it passageway or intersection, roll 2d6. On a result of 9 or higher, an Encounter occurs. At that point, roll 3d6 against the Sewer Encounter Table to see what turns up.
Die Roll SEWER ENCOUNTER TABLE
(3D6) Critters
3 1 large, rabid cat
4 1 large, rabid dog
5 1-6 bats
6 1-6 ghouls
7-8 1 - 6 Street Gang members•
9-10 1 - 6 Squatters••
11- 12 1-6 Orks•••
13-14 1-6 Dwarfs
15 1 or 2 ghosts
16 2- 12 rats
17 1-2 ghosts
18 1 vampire

"Street Gang: One of the Gang Members present is the leader.
Normally, he would be a Gang Leader Archetype, but roll 106.
On a result of 5, he is a Street Samurai; on a result of 6, he is a
Street Mage.
••Squatters: Roll 206: On a roll of 2 or 3, there is a classic Street
Archetype concealed among the Squatters. Roll 1 06: 1 Is a
Street Shaman; 2 Street Mage; 3 Gang Member; 4 - 5 Street
Samurai, and 6 Street Doc. Otherwise, all are the standard
Squatter Archetype.
•••orks: These are local, sewer-dwelling Orks. They are harmless,
though it may not be immediately apparent. Use the
Squatter Archetype. These Orks have no connection with the
hidden Ork enclave encountered later in the adventure.
For all other encounters, use the statistics given for the
appropriate Critter or Average Metahuman, per the tables on
page 191-2 of the Shadowrun rules. All appropriate skills are
at a Rating of 2.