How does chronic stress improve memory processes? There seems to be more information on the detrimental effects of stress on memory.

There is evidence to the contrary.

Stress effects on memory: an update and integration.
Schwabe L, et al  doi: 10.1016/j.neubiorev.2011.07.002


  It is well known that stressful experiences may affect learning and memory processes. Less clear is the exact nature of these stress effects on memory: both enhancing and impairing effects have been reported.


I am interested in the research available on stress positively effecting memory.

What are the biological reasons for stress enhancing memory processes?

There is a related question Can the brain work better in stressful conditions?; I am interested more in chronic stress. An examination of the effects of acute stress on memory, I believe, warrants a separate discussion.
Interesting question. Intuitively, I would have thought that chronic stress would uncompromisingly be a detriment to our memory and recall.

An article by Ohira et al (2011) makes note that it is accepted that chronic stress is seen to adversely affect memory, it may not be replicated in other studies:


  "However, results related to the effects of chronic job stress
  and cognitive functions remain controversial. Although
  chronic job stress is generally linked to reduced memory
  and learning (Holman and Wall, 2002; Taris and Feij,
  2004), a recent epidemiological study with a larger sample
  size failed to replicate these effects (Elovainio et al., 2009)."


Other studies may indicate that predictable chronic mild stress can improve hippocampal neurogenesis and memory (Parihar et al., 2011). Your question doesn't describe the level of stress experienced as a variable so broadly, this article might be very relevant:


  "Furthermore, enrichments in mood and memory after PCMS were
  associated with substantially increased hippocampal neurogenesis ...
  The overall stress reaction in an organism can be advantageous for
  cognitive functions when it induces moderate increases in
  corticosterone levels and leads to adaptation and resistance ... The
  PCMS regimen used in this study likely elicited moderate increases in
  corticosterone levels, which facilitated adaptive and beneficial
  responses in the brain."


It's implied that unpredictable chronic stress would have a detrimental effect on memory. Some of the biological reasons for enhancing memory processes are presented in that above study.

EDIT: Additional Information

Also see - McFadden, L.M., Paris, J.J., Mitzelfelt, M.S., McDonough, S., Frye, C.A. &amp; Matuszewich, L. (2011). Sex-dependent effects of chronic unpredictable stress in the water maze. Physiology &amp; Behavior, 102(3), 266-275. The research conversely highlights that unpredictable stress improves performance on a spatial memory task in female rats.

References


Ohira, H., Matsunga, M., Kumra, K., Murakami, H., Osumi., T., Isowa, T., Fukuyama, S., Shinoda, J. &amp; Yamada, J. (2011). Chronic stress modulates neural and cardiovascular responses during reversal learning. Neuroscience, 193, 193-204
Parihar, VK., Hattiangady, B., Kuruba, R., Shuai, B. &amp; Shetty, AK. (2011). Predictable chronic mild stress improves mood, hippocampal neurogenesis and memory. Molecular Psychiatry, 16, 171-183