July 16 (Reuters) - Amazon.com Inc-backed (AMZN.O) electric-vehicle startup Rivian Automotive will delay the deliveries of its debut vehicle by more than a month due to supply chain issues, according to a letter written by its CEO to customers. Rivian, seen as a potential Tesla Inc (TSLA.O) rival, said the timing for the first deliveries of its R1T pickup initially slated for July has now been pushed to September, while that of R1S SUVs have been delayed until the fall. "The cascading impacts of the pandemic have had a compounding effect greater than anyone anticipated," Chief Executive Officer R.J. Scaringe said in a letter reviewed by Reuters. "Everything from facility construction, to equipment installation, to vehicle component supply has been impacted." The global semiconductor crunch has forced carmakers including Volkswagen (VOWG_p.DE), Ford (F.N) and General Motors (GM.N) to cut output as the chip market was swept clean by makers of consumer electronics. California-based Rivian, also backed by Ford, is looking to go public later this year and could target a valuation of about $70 billion, Bloomberg News reported in May. read more