This is a more technical word for removing rocks, mud and any material from a riverbed, channel, etc. There is a کف بستر in the text which gives me a notion that the excavation is happening where there is water!
1. (Engineering / Mechanical Engineering) to remove (material) from a riverbed, channel, etc., by means of a dredge

2. (Engineering / Mechanical Engineering) (tr) to search for (a submerged object) with or as if with a dredge; drag

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Note added at 8 hrs (2013-10-13 18:24:13 GMT)
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"Removing bedrock" would be a better translation for سنگبرداری کف بستر . Please see the link below:
http://www.legalrecord.net/state-news/7131-crews-removing-mi...