If you only have certain sections that need extended you could only select those river segements to buffer.  The buffer result polygon could be converted to a polyline.  Next, you could update the old section layer with the buffer river layer by using the Erase Tool.  Finally, merge or append the new section polyline features with the result of the erased polyline section layer.  This should extend your section layer and eliminate the old section lines.

ArcInfo - How to extend batch of lines to another object
Suppose that I have an AES key $K$, and I'm instructed to find a plaintext such that the first 32 bits of the plaintext are some string of bits $S_1$, and the last 32 bits of the ciphertext once the plaintext has been encrypted with $K$ are another string of bits $S_2$.

Is this difficult? Is there a known attack against this that's faster than just fixing one of the $S$'s and brute-force searching for a text with the other $S$?
