As the time streem, by definition, is a place where time travellers from all epochs arrive, it has no actual chronology.
The time streem first appears in THE MARRISSA GAMES, as Agents J and K travel through it to get to Portal High School. It also appears in HARRY POTTER AN THE KILL OF SNAPE, where Robot and Roslalonde Lettuce meet Skepness Man there and entrust him with his mission.
There appear to be no permanent structures in the time streem. However, it is known that there is at least breathable air in the time streem, as J and K are able to traverse it in a motorcycle without any special equipment.
The time streem is not always invoked in MarissaTheWriter stories; in some, like ITS MY LIFE! and Post-SCrash Session 3: Spectators of the Host, the time travel is instantaneous and can get someone from point A to point B without a medium.