First read Stephen Gornick's link. The short answer is yes, you are obligated to report all sources of income.
If the government picks your return for an audit (based in part on your 'paper trail' of extra income, or just a random luck of the draw), you don't want to be in the position of explaining why you didn't report it. In the end the choice of reporting it is yours, but you should understand the consequences of not doing so. This applies to both US and Canada.
One benefit of reporting mining income is that you can deduct all reasonable expenses associated with it (such as depreciation on your mining rigs, electricity, etc.).