The hidden story of the River Sheaf Bridges in Sheffield

In 1852  the lower reaches of the River Sheaf in Sheffield (UK) were crossed only by the Sheaf Bridge and Canal Bridge close to the Corn Exchange,  a footbridge at the foot of Pond Hill, and the ‘Farm Bridge’ at the junction of Shrewsbury Road, Suffolk Road, St. Mary’s Road and Queens Road. 

This stretch of the river, almost 1.5km. long, now runs in culverts (except for one 40m. exposed section near Pond Hill) and emerges into the River Don under Castlegate at its junction with Blonk Street (Grid Ref. SK35858773).

The River Sheaf’s junction with the Don, the arch on the right, 
with Blonk Street Bridge, seen on the left.
(From Wikipedia)

The building of the culvert, now known to Urban Explorers as “The Megatron”, started in about 1850 when the section between the Sheaf Bridge (on Broad Street), and the Canal Bridge (on Exchange Street) was covered to  create the Castlefolds Market which stood between the existing Corn Exchange and the new Norfolk Market Hall.  Later sections were added over the years and the Sheaf Bridge and Canal Bridge were lost and forgotten.

But are they still there buried in the culvert?  This posting gives a glimpse into some research I did recently to try and untangle the story of the culvert and to try and find the old bridges.

Did a bit of Hartshead, Sheffield, end up in Washington DC?

Did an American Journalist make a pilgrimage to Sheffield (UK) in 1868 to find the shop where his old employer was born amid the radical ferment of the late 18th century?

Did he buy the old frontage of the shop, have it made into a couple of boxes, and donate one to the National Museum in Washington DC (as in USA)?

It's a great story, and it has been interesting trying to follow the trail.  Here is what I think I know about the story ..... (spoiler! I still don't know the answer to all of the above questions!)