The Avon Canoe Pilot - Dredging.

Navigable waterways need to be kept clear of sediment and hazardous obstructions.

Dredging is one such cleaning method, but is also shopping for the frugally 
minded. It gives an element of the unexpected and has a 100% return on any 

Use a grappling hook attached to a length of rope. The rope length needs to 
surpass the depth of the specified waters. Apply a simple method of launching the 
tool into the water, allowing some moments to pass as it settles, then slowly reel 
it in.

What has been lost and discarded, might find its way onto the end of your line.
Certain areas promise certain possibilities: 

Shopping trolleys are ubiquitous. Skateboards can be found near the waters of 
Lloyds building skating spot, bicycles at Vauxhall and Bedminster Bridges and 
motorbikes at Cumberland Basin.
Dredging off vauxhall bridge and lloyds amphitheatre.

The Avon Canoe Pilot.