MORNING UPDATE: The Muddy River dropped below flood stage, even if barely, so Riverside service is running this morning - although river levels have started rising again (hourly river-level data).
LATER MORNING UPDATE: The Muddy River got all floody again, so the T put the dam back and stopped Riverside service.
Around 9 p.m., the MBTA blocked off the Fenway portal used by the Riverside line to prevent a recurrence of the 1996 floods that left Kenmore station submerged in 20 feet of water and required $40 million in repairs.
The move means Riverside commuters can expect delays tomorrow as they ride shuttle buses instead of trolleys between Reservoir and Kenmore, the T advises.
A federal monitoring station on the Muddy reported the normally docile river reached flood stage around 8:45 p.m. A T alert announcing the suspension of service between Kenmore and Reservoir went out not long after.
Following a 1962 flood, the then MTA built some columns, near where the Riverside line goes underground, into which to fit wooden boards to act as a temporary dam. One of the reasons for the 1996 flooding was that workers couldn't find the boards in time and, as in 1962, sandbags failed to keep the Muddy River out of the station.
USGS flood chart at 8:45 p.m.:
Thanks to the folks in this thread for their info.