After doing a little digging around, it looks like there's a fair degree of consensus that all Bitcoin Cash (BCH) generated from the fork is considered income for owners of Bitcoin (BTC) private keys, as of August 1, and this income is reportable in 2017, whether you actually take control of the new BCH, to sell it or trade it, or not.
https://www.forbes.com/sites/greatspeculations/2017/08/04/how-to-report-bitcoin-cash-and-avoid-irs-trouble/#65759e173066https://bitcoin.tax/blog/how-to-tax-bitcoin-cash-bch/Forbes is using an August 1 price of $266 and Bitcoin.tax is using $277 as the initial BCH basis.
So, for example, 10 BTC held in private keys you controlled on July 31 generates 10 BCH on August 1 with a value of $2770 that must be reported as income in 2017, even if you don't do anything with it in 2017. Even if you don't know how to actually use your BTC keys to gain control of your newfound BCH wealth, you still owe the tax man.
If you sell for USD or exchange the new BCH for another crypto this year, then capital gains applies. So, 10 BCH, valued at $5770, exchanged for 1.3 BTC on August 18 produces $3000 of short-term gain, which is taxable as regular income.
It looks like any BTC held at Coinbase on August 1 won't produce any reportable BCH income in 2017, because Coinbase isn't making those BCH available to account holders until January 1. But holdings at Blockchain.info on August 1 is reportable because account holders retain control over their private keys (even if they don't know it), plus they made it possible to hit a button and transfer out BCH holdings last month.
It's really weird to be paying taxes on something you had no control over acquiring. And what about the poor suckers who got scammed into thinking they were redeeming BCH but had their BCT and BCH both stolen in the process?
And then there's the more recent Bitcoin Gold fork, too. It forked weeks before you could even take control of it and there's still hardly anybody exchanging it.
Crap, this little development isn't going to do anything good for my marginal tax bracket, but I don't see any legal way out of it. The blockchain is immutable, its transactions are searchable, and with enough motivation its addresses can be linked back to meat space.
Gotta pay to play if you ever plan on converting substantial amounts of crypto into fiat.